diff --git a/NEWS.md b/NEWS.md
index e33408054ea5e4..96da441a627813 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -197,7 +197,7 @@ The following default gems are updated.
 * fileutils 1.8.0
 * io-console 0.8.1
 * io-nonblock 0.3.2
-* io-wait 0.3.5
+* io-wait 0.4.0.dev
 * json 2.16.0
 * net-http 0.7.0
 * openssl 4.0.0.pre
diff --git a/ext/io/wait/extconf.rb b/ext/io/wait/extconf.rb
index e97efa179d29c7..00c455a45c8f6f 100644
--- a/ext/io/wait/extconf.rb
+++ b/ext/io/wait/extconf.rb
@@ -1,21 +1,4 @@
 # frozen_string_literal: false
 require 'mkmf'
 
-target = "io/wait"
-have_func("rb_io_wait", "ruby/io.h")
-have_func("rb_io_descriptor", "ruby/io.h")
-unless macro_defined?("DOSISH", "#include <ruby.h>")
-  have_header(ioctl_h = "sys/ioctl.h") or ioctl_h = nil
-  fionread = %w[sys/ioctl.h sys/filio.h sys/socket.h].find do |h|
-    have_macro("FIONREAD", [h, ioctl_h].compact)
-  end
-  if fionread
-    $defs << "-DFIONREAD_HEADER=\"<#{fionread}>\""
-    create_makefile(target)
-  end
-else
-  if have_func("rb_w32_ioctlsocket", "ruby.h")
-    have_func("rb_w32_is_socket", "ruby.h")
-    create_makefile(target)
-  end
-end
+create_makefile("io/wait")
diff --git a/ext/io/wait/io-wait.gemspec b/ext/io/wait/io-wait.gemspec
index 3d75aab9c19b09..ef46fbca0f6930 100644
--- a/ext/io/wait/io-wait.gemspec
+++ b/ext/io/wait/io-wait.gemspec
@@ -1,4 +1,4 @@
-_VERSION = "0.3.5"
+_VERSION = "0.4.0.dev"
 
 Gem::Specification.new do |spec|
   spec.name          = "io-wait"
@@ -10,7 +10,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Waits until IO is readable or writable without blocking.}
   spec.homepage      = "https://github.com/ruby/io-wait"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
-  spec.required_ruby_version = Gem::Requirement.new(">= 3.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 3.2")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
diff --git a/ext/io/wait/wait.c b/ext/io/wait/wait.c
index bec04cb727e054..f27d7c11e5db48 100644
--- a/ext/io/wait/wait.c
+++ b/ext/io/wait/wait.c
@@ -11,427 +11,13 @@
 
 **********************************************************************/
 
-#include "ruby.h"
-#include "ruby/io.h"
-
-#include <sys/types.h>
-#if defined(HAVE_UNISTD_H) && (defined(__sun))
-#include <unistd.h>
-#endif
-#if defined(HAVE_SYS_IOCTL_H)
-#include <sys/ioctl.h>
-#endif
-#if defined(FIONREAD_HEADER)
-#include FIONREAD_HEADER
-#endif
-
-#ifdef HAVE_RB_W32_IOCTLSOCKET
-#define ioctl ioctlsocket
-#define ioctl_arg u_long
-#define ioctl_arg2num(i) ULONG2NUM(i)
-#else
-#define ioctl_arg int
-#define ioctl_arg2num(i) INT2NUM(i)
-#endif
-
-#ifdef HAVE_RB_W32_IS_SOCKET
-#define FIONREAD_POSSIBLE_P(fd) rb_w32_is_socket(fd)
-#else
-#define FIONREAD_POSSIBLE_P(fd) ((void)(fd),Qtrue)
-#endif
-
-#ifndef HAVE_RB_IO_WAIT
-static struct timeval *
-get_timeout(int argc, VALUE *argv, struct timeval *timerec)
-{
-    VALUE timeout = Qnil;
-    rb_check_arity(argc, 0, 1);
-    if (!argc || NIL_P(timeout = argv[0])) {
-        return NULL;
-    }
-    else {
-        *timerec = rb_time_interval(timeout);
-        return timerec;
-    }
-}
-
-static int
-wait_for_single_fd(rb_io_t *fptr, int events, struct timeval *tv)
-{
-    int i = rb_wait_for_single_fd(fptr->fd, events, tv);
-    if (i < 0)
-        rb_sys_fail(0);
-    rb_io_check_closed(fptr);
-    return (i & events);
-}
-#endif
-
-/*
- * call-seq:
- *   io.nread -> int
- *
- * Returns number of bytes that can be read without blocking.
- * Returns zero if no information available.
- *
- * You must require 'io/wait' to use this method.
- */
-
-static VALUE
-io_nread(VALUE io)
-{
-    rb_io_t *fptr;
-    int len;
-    ioctl_arg n;
-
-    GetOpenFile(io, fptr);
-    rb_io_check_char_readable(fptr);
-    len = rb_io_read_pending(fptr);
-    if (len > 0) return INT2FIX(len);
-
-#ifdef HAVE_RB_IO_DESCRIPTOR
-    int fd = rb_io_descriptor(io);
-#else
-    int fd = fptr->fd;
-#endif
-
-    if (!FIONREAD_POSSIBLE_P(fd)) return INT2FIX(0);
-    if (ioctl(fd, FIONREAD, &n)) return INT2FIX(0);
-    if (n > 0) return ioctl_arg2num(n);
-    return INT2FIX(0);
-}
-
-#ifdef HAVE_RB_IO_WAIT
-static VALUE
-io_wait_event(VALUE io, int event, VALUE timeout, int return_io)
-{
-    VALUE result = rb_io_wait(io, RB_INT2NUM(event), timeout);
-
-    if (!RB_TEST(result)) {
-        return Qnil;
-    }
-
-    int mask = RB_NUM2INT(result);
-
-    if (mask & event) {
-        if (return_io)
-            return io;
-        else
-            return result;
-    }
-    else {
-        return Qfalse;
-    }
-}
-#endif
-
-/*
- * call-seq:
- *   io.ready? -> truthy or falsy
- *
- * Returns a truthy value if input available without blocking, or a
- * falsy value.
- *
- * You must require 'io/wait' to use this method.
- */
-
-static VALUE
-io_ready_p(VALUE io)
-{
-    rb_io_t *fptr;
-#ifndef HAVE_RB_IO_WAIT
-    struct timeval tv = {0, 0};
-#endif
-
-    GetOpenFile(io, fptr);
-    rb_io_check_char_readable(fptr);
-    if (rb_io_read_pending(fptr)) return Qtrue;
-
-#ifndef HAVE_RB_IO_WAIT
-    return wait_for_single_fd(fptr, RB_WAITFD_IN, &tv) ? Qtrue : Qfalse;
-#else
-    return io_wait_event(io, RUBY_IO_READABLE, RB_INT2NUM(0), 1);
-#endif
-}
-
-/* Ruby 3.2+ can define these methods. This macro indicates that case. */
-#ifndef RUBY_IO_WAIT_METHODS
-
-/*
- * call-seq:
- *   io.wait_readable          -> truthy or falsy
- *   io.wait_readable(timeout) -> truthy or falsy
- *
- * Waits until IO is readable and returns a truthy value, or a falsy
- * value when times out.  Returns a truthy value immediately when
- * buffered data is available.
- *
- * You must require 'io/wait' to use this method.
- */
-
-static VALUE
-io_wait_readable(int argc, VALUE *argv, VALUE io)
-{
-    rb_io_t *fptr;
-#ifndef HAVE_RB_IO_WAIT
-    struct timeval timerec;
-    struct timeval *tv;
-#endif
-
-    GetOpenFile(io, fptr);
-    rb_io_check_char_readable(fptr);
-
-#ifndef HAVE_RB_IO_WAIT
-    tv = get_timeout(argc, argv, &timerec);
-#endif
-    if (rb_io_read_pending(fptr)) return Qtrue;
-
-#ifndef HAVE_RB_IO_WAIT
-    if (wait_for_single_fd(fptr, RB_WAITFD_IN, tv)) {
-        return io;
-    }
-    return Qnil;
-#else
-    rb_check_arity(argc, 0, 1);
-    VALUE timeout = (argc == 1 ? argv[0] : Qnil);
-
-    return io_wait_event(io, RUBY_IO_READABLE, timeout, 1);
-#endif
-}
-
-/*
- * call-seq:
- *   io.wait_writable          -> truthy or falsy
- *   io.wait_writable(timeout) -> truthy or falsy
- *
- * Waits until IO is writable and returns a truthy value or a falsy
- * value when times out.
- *
- * You must require 'io/wait' to use this method.
- */
-static VALUE
-io_wait_writable(int argc, VALUE *argv, VALUE io)
-{
-    rb_io_t *fptr;
-#ifndef HAVE_RB_IO_WAIT
-    struct timeval timerec;
-    struct timeval *tv;
-#endif
-
-    GetOpenFile(io, fptr);
-    rb_io_check_writable(fptr);
-
-#ifndef HAVE_RB_IO_WAIT
-    tv = get_timeout(argc, argv, &timerec);
-    if (wait_for_single_fd(fptr, RB_WAITFD_OUT, tv)) {
-        return io;
-    }
-    return Qnil;
-#else
-    rb_check_arity(argc, 0, 1);
-    VALUE timeout = (argc == 1 ? argv[0] : Qnil);
-
-    return io_wait_event(io, RUBY_IO_WRITABLE, timeout, 1);
-#endif
-}
-
-#ifdef HAVE_RB_IO_WAIT
-/*
- * call-seq:
- *   io.wait_priority          -> truthy or falsy
- *   io.wait_priority(timeout) -> truthy or falsy
- *
- * Waits until IO is priority and returns a truthy value or a falsy
- * value when times out. Priority data is sent and received using
- * the Socket::MSG_OOB flag and is typically limited to streams.
- *
- * You must require 'io/wait' to use this method.
- */
-static VALUE
-io_wait_priority(int argc, VALUE *argv, VALUE io)
-{
-    rb_io_t *fptr = NULL;
-
-    RB_IO_POINTER(io, fptr);
-    rb_io_check_char_readable(fptr);
-
-    if (rb_io_read_pending(fptr)) return Qtrue;
-
-    rb_check_arity(argc, 0, 1);
-    VALUE timeout = argc == 1 ? argv[0] : Qnil;
-
-    return io_wait_event(io, RUBY_IO_PRIORITY, timeout, 1);
-}
-#endif
-
-static int
-wait_mode_sym(VALUE mode)
-{
-    if (mode == ID2SYM(rb_intern("r"))) {
-        return RB_WAITFD_IN;
-    }
-    if (mode == ID2SYM(rb_intern("read"))) {
-        return RB_WAITFD_IN;
-    }
-    if (mode == ID2SYM(rb_intern("readable"))) {
-        return RB_WAITFD_IN;
-    }
-    if (mode == ID2SYM(rb_intern("w"))) {
-        return RB_WAITFD_OUT;
-    }
-    if (mode == ID2SYM(rb_intern("write"))) {
-        return RB_WAITFD_OUT;
-    }
-    if (mode == ID2SYM(rb_intern("writable"))) {
-        return RB_WAITFD_OUT;
-    }
-    if (mode == ID2SYM(rb_intern("rw"))) {
-        return RB_WAITFD_IN|RB_WAITFD_OUT;
-    }
-    if (mode == ID2SYM(rb_intern("read_write"))) {
-        return RB_WAITFD_IN|RB_WAITFD_OUT;
-    }
-    if (mode == ID2SYM(rb_intern("readable_writable"))) {
-        return RB_WAITFD_IN|RB_WAITFD_OUT;
-    }
-    rb_raise(rb_eArgError, "unsupported mode: %"PRIsVALUE, mode);
-    return 0;
-}
-
-#ifdef HAVE_RB_IO_WAIT
-static inline rb_io_event_t
-io_event_from_value(VALUE value)
-{
-    int events = RB_NUM2INT(value);
-
-    if (events <= 0) rb_raise(rb_eArgError, "Events must be positive integer!");
-
-    return events;
-}
-#endif
-
 /*
- * call-seq:
- *   io.wait(events, timeout) -> event mask, false or nil
- *   io.wait(*event_symbols[, timeout]) -> self, true, or false
- *
- * Waits until the IO becomes ready for the specified events and returns the
- * subset of events that become ready, or a falsy value when times out.
- *
- * The events can be a bit mask of +IO::READABLE+, +IO::WRITABLE+ or
- * +IO::PRIORITY+.
- *
- * Returns an event mask (truthy value) immediately when buffered data is
- * available.
- *
- * The second form: if one or more event symbols (+:read+, +:write+, or
- * +:read_write+) are passed, the event mask is the bit OR of the bitmask
- * corresponding to those symbols.  In this form, +timeout+ is optional, the
- * order of the arguments is arbitrary, and returns +io+ if any of the
- * events is ready.
- *
- * You must require 'io/wait' to use this method.
+ * IO wait methods are built in ruby now, just for backward compatibility.
  */
 
-static VALUE
-io_wait(int argc, VALUE *argv, VALUE io)
-{
-#ifndef HAVE_RB_IO_WAIT
-    rb_io_t *fptr;
-    struct timeval timerec;
-    struct timeval *tv = NULL;
-    int event = 0;
-    int i;
-
-    GetOpenFile(io, fptr);
-    for (i = 0; i < argc; ++i) {
-        if (SYMBOL_P(argv[i])) {
-            event |= wait_mode_sym(argv[i]);
-        }
-        else {
-            *(tv = &timerec) = rb_time_interval(argv[i]);
-        }
-    }
-    /* rb_time_interval() and might_mode() might convert the argument */
-    rb_io_check_closed(fptr);
-    if (!event) event = RB_WAITFD_IN;
-    if ((event & RB_WAITFD_IN) && rb_io_read_pending(fptr))
-        return Qtrue;
-    if (wait_for_single_fd(fptr, event, tv))
-        return io;
-    return Qnil;
-#else
-    VALUE timeout = Qundef;
-    rb_io_event_t events = 0;
-    int i, return_io = 0;
-
-    if (argc != 2 || (RB_SYMBOL_P(argv[0]) || RB_SYMBOL_P(argv[1]))) {
-        /* We'd prefer to return the actual mask, but this form would return the io itself: */
-        return_io = 1;
-
-        /* Slow/messy path: */
-        for (i = 0; i < argc; i += 1) {
-            if (RB_SYMBOL_P(argv[i])) {
-                events |= wait_mode_sym(argv[i]);
-            }
-            else if (timeout == Qundef) {
-                rb_time_interval(timeout = argv[i]);
-            }
-            else {
-                rb_raise(rb_eArgError, "timeout given more than once");
-            }
-        }
-
-        if (timeout == Qundef) timeout = Qnil;
-
-        if (events == 0) {
-            events = RUBY_IO_READABLE;
-        }
-    }
-    else /* argc == 2 and neither are symbols */ {
-        /* This is the fast path: */
-        events = io_event_from_value(argv[0]);
-        timeout = argv[1];
-    }
-
-    if (events & RUBY_IO_READABLE) {
-        rb_io_t *fptr = NULL;
-        RB_IO_POINTER(io, fptr);
-
-        if (rb_io_read_pending(fptr)) {
-            /* This was the original behaviour: */
-            if (return_io) return Qtrue;
-            /* New behaviour always returns an event mask: */
-            else return RB_INT2NUM(RUBY_IO_READABLE);
-        }
-    }
-
-    return io_wait_event(io, events, timeout, return_io);
-#endif
-}
-
-#endif /* RUBY_IO_WAIT_METHODS */
-
-/*
- * IO wait methods
- */
+#include "ruby.h"
 
 void
 Init_wait(void)
 {
-#ifdef HAVE_RB_EXT_RACTOR_SAFE
-    RB_EXT_RACTOR_SAFE(true);
-#endif
-
-    rb_define_method(rb_cIO, "nread", io_nread, 0);
-    rb_define_method(rb_cIO, "ready?", io_ready_p, 0);
-
-#ifndef RUBY_IO_WAIT_METHODS
-    rb_define_method(rb_cIO, "wait", io_wait, -1);
-
-    rb_define_method(rb_cIO, "wait_readable", io_wait_readable, -1);
-    rb_define_method(rb_cIO, "wait_writable", io_wait_writable, -1);
-#ifdef HAVE_RB_IO_WAIT
-    rb_define_method(rb_cIO, "wait_priority", io_wait_priority, -1);
-#endif
-#endif
 }
diff --git a/test/io/wait/test_io_wait.rb b/test/io/wait/test_io_wait.rb
index 445363714c0912..ffe3b1f760a626 100644
--- a/test/io/wait/test_io_wait.rb
+++ b/test/io/wait/test_io_wait.rb
@@ -4,9 +4,6 @@
 require 'timeout'
 require 'socket'
 
-# For `IO#ready?` and `IO#nread`:
-require 'io/wait'
-
 class TestIOWait < Test::Unit::TestCase
 
   def setup
@@ -22,33 +19,6 @@ def teardown
     @w.close unless @w.closed?
   end
 
-  def test_nread
-    assert_equal 0, @r.nread
-    @w.syswrite "."
-    sleep 0.1
-    assert_equal 1, @r.nread
-  end
-
-  def test_nread_buffered
-    @w.syswrite ".\n!"
-    assert_equal ".\n", @r.gets
-    assert_equal 1, @r.nread
-  end
-
-  def test_ready?
-    omit 'unstable on MinGW' if /mingw/ =~ RUBY_PLATFORM
-    assert_not_predicate @r, :ready?, "shouldn't ready, but ready"
-    @w.syswrite "."
-    sleep 0.1
-    assert_predicate @r, :ready?, "should ready, but not"
-  end
-
-  def test_buffered_ready?
-    @w.syswrite ".\n!"
-    assert_equal ".\n", @r.gets
-    assert_predicate @r, :ready?
-  end
-
   def test_wait
     omit 'unstable on MinGW' if /mingw/ =~ RUBY_PLATFORM
     assert_nil @r.wait(0)
diff --git a/test/io/wait/test_io_wait_uncommon.rb b/test/io/wait/test_io_wait_uncommon.rb
index 0f97ac35d9124e..15b13b51b1001a 100644
--- a/test/io/wait/test_io_wait_uncommon.rb
+++ b/test/io/wait/test_io_wait_uncommon.rb
@@ -76,14 +76,6 @@ def test_wait_writable_null
     check_dev(IO::NULL, :wait_writable)
   end
 
-  def test_after_ungetc_ready?
-    check_dev(IO::NULL, mode: "r") {|fp|
-      assert_respond_to fp, :ready?
-      fp.ungetc(?a)
-      assert_predicate fp, :ready?
-    }
-  end
-
   def test_after_ungetc_wait_readable
     check_dev(IO::NULL, mode: "r") {|fp|
       fp.ungetc(?a)
@@ -91,13 +83,6 @@ def test_after_ungetc_wait_readable
     }
   end
 
-  def test_after_ungetc_in_text_ready?
-    check_dev(IO::NULL, mode: "rt") {|fp|
-      fp.ungetc(?a)
-      assert_predicate fp, :ready?
-    }
-  end
-
   def test_after_ungetc_in_text_wait_readable
     check_dev(IO::NULL, mode: "rt") {|fp|
       fp.ungetc(?a)
diff --git a/test/rubygems/test_gem_package_tar_header_ractor.rb b/test/rubygems/test_gem_package_tar_header_ractor.rb
index 6f417f43f90cd2..bcd8ab6ad9a603 100644
--- a/test/rubygems/test_gem_package_tar_header_ractor.rb
+++ b/test/rubygems/test_gem_package_tar_header_ractor.rb
@@ -1,6 +1,12 @@
 # frozen_string_literal: true
+
 require_relative "package/tar_test_case"
 
+unless Gem::Package::TarTestCase.instance_methods.include?(:assert_ractor)
+  require "core_assertions"
+  Gem::Package::TarTestCase.include Test::Unit::CoreAssertions
+end
+
 class TestGemPackageTarHeaderRactor < Gem::Package::TarTestCase
   ASSERT_HEADERS_EQUAL = <<~RUBY
     def assert_headers_equal(expected, actual)
@@ -65,10 +71,14 @@ def assert_headers_equal(expected, actual)
     }
 
     tar_header = Gem::Package::TarHeader.new header
+    # Move this require to arguments of assert_ractor after Ruby 4.0 or updating core_assertions.rb at Ruby 3.4.
+    require "stringio"
+    # Remove this after Ruby 4.0 or updating core_assertions.rb at Ruby 3.4.
+    class Ractor; alias value take unless method_defined?(:value); end
   RUBY
 
   def test_decode_in_ractor
-    assert_ractor(ASSERT_HEADERS_EQUAL + SETUP + <<~RUBY, require: ["rubygems/package", "stringio"])
+    assert_ractor(ASSERT_HEADERS_EQUAL + SETUP + <<~RUBY, require: "rubygems/package")
       new_header = Ractor.new(tar_header.to_s) do |str|
         Gem::Package::TarHeader.from StringIO.new str
       end.value
@@ -78,7 +88,7 @@ def test_decode_in_ractor
   end
 
   def test_encode_in_ractor
-    assert_ractor(ASSERT_HEADERS_EQUAL + SETUP + <<~RUBY, require: ["rubygems/package", "stringio"])
+    assert_ractor(ASSERT_HEADERS_EQUAL + SETUP + <<~RUBY, require: "rubygems/package")
       header_bytes = tar_header.to_s
 
       new_header_bytes = Ractor.new(header_bytes) do |str|
diff --git a/tool/bundler/dev_gems.rb b/tool/bundler/dev_gems.rb
index 0039a42171aa41..91ac5628f1f30c 100644
--- a/tool/bundler/dev_gems.rb
+++ b/tool/bundler/dev_gems.rb
@@ -3,6 +3,7 @@
 source "https://rubygems.org"
 
 gem "test-unit", "~> 3.0"
+gem "test-unit-ruby-core"
 gem "rake", "~> 13.1"
 gem "rb_sys"
 
diff --git a/tool/bundler/dev_gems.rb.lock b/tool/bundler/dev_gems.rb.lock
index 7f73c43f4cc1dd..f2a5245ede239a 100644
--- a/tool/bundler/dev_gems.rb.lock
+++ b/tool/bundler/dev_gems.rb.lock
@@ -59,6 +59,8 @@ GEM
       compact_index (~> 0.15.0)
     test-unit (3.6.7)
       power_assert
+    test-unit-ruby-core (1.0.13)
+      test-unit
     turbo_tests (2.2.5)
       parallel_tests (>= 3.3.0, < 5)
       rspec (>= 3.10)
@@ -88,6 +90,7 @@ DEPENDENCIES
   rspec-mocks (~> 3.12)
   rubygems-generate_index (~> 1.1)
   test-unit (~> 3.0)
+  test-unit-ruby-core
   turbo_tests (~> 2.2.3)
 
 CHECKSUMS
@@ -122,6 +125,7 @@ CHECKSUMS
   rspec-support (3.13.2) sha256=cea3a2463fd9b84b9dcc9685efd80ea701aa8f7b3decb3b3ce795ed67737dbec
   rubygems-generate_index (1.1.3) sha256=3571424322666598e9586a906485e1543b617f87644913eaf137d986a3393f5c
   test-unit (3.6.7) sha256=c342bb9f7334ea84a361b43c20b063f405c0bf3c7dbe3ff38f61a91661d29221
+  test-unit-ruby-core (1.0.13) sha256=0c08e61b3b97060028a47d8fc4827de077fdbfc26ef80b4afd9035b9e15ecc5b
   turbo_tests (2.2.5) sha256=3fa31497d12976d11ccc298add29107b92bda94a90d8a0a5783f06f05102509f
 
 BUNDLED WITH