diff --git a/src/main/java/org/jruby/ext/openssl/SSLSocket.java b/src/main/java/org/jruby/ext/openssl/SSLSocket.java
index 0a2f801e..661c0b6d 100644
--- a/src/main/java/org/jruby/ext/openssl/SSLSocket.java
+++ b/src/main/java/org/jruby/ext/openssl/SSLSocket.java
@@ -688,7 +688,7 @@ public int write(ByteBuffer src, boolean blocking) throws SSLException, IOExcept
             if ( netWriteData.hasRemaining() ) {
                 flushData(blocking);
             }
-            netWriteData.clear();
+            netWriteData.compact();
             final SSLEngineResult result = engine.wrap(src, netWriteData);
             if ( result.getStatus() == SSLEngineResult.Status.CLOSED ) {
                 throw getRuntime().newIOError("closed SSL engine");
@@ -830,6 +830,12 @@ private IRubyObject sysreadImpl(final ThreadContext context, final IRubyObject l
         }
 
         try {
+            // Flush any pending encrypted write data before reading, so the
+            // remote side receives the complete request before we wait for a response.
+            if ( engine != null && netWriteData.hasRemaining() ) {
+                flushData(true);
+            }
+
             // So we need to make sure to only block when there is no data left to process
             if ( engine == null || ! ( appReadData.hasRemaining() || netReadData.position() > 0 ) ) {
                 final Object ex = waitSelect(SelectionKey.OP_READ, blocking, exception);
diff --git a/src/test/ruby/ssl/test_write_flush.rb b/src/test/ruby/ssl/test_write_flush.rb
new file mode 100644
index 00000000..e6ce817b
--- /dev/null
+++ b/src/test/ruby/ssl/test_write_flush.rb
@@ -0,0 +1,81 @@
+# frozen_string_literal: false
+require File.expand_path('test_helper', File.dirname(__FILE__))
+
+class TestSSLWriteFlush < TestCase
+
+  include SSLTestHelper
+
+  # write_nonblock a large payload then read the server's response.
+  #
+  # This exercises the write -> read transition used by net/http for POST
+  # requests. Two bugs in SSLSocket caused data loss here:
+  #   1. write() called netWriteData.clear() after a partial non-blocking
+  #      flush, discarding encrypted bytes not yet sent to the socket.
+  #   2. sysreadImpl() did not flush pending netWriteData before reading.
+  #
+  # Without the fix the TLS stream is corrupted and the connection breaks
+  # with EPIPE.
+  def test_write_nonblock_then_read
+    data_size = 256 * 1024
+    data = "X" * data_size
+
+    server_proc = proc { |ctx, ssl|
+      begin
+        received = ""
+        begin
+          while received.bytesize < data_size
+            received << ssl.readpartial(8192)
+          end
+        rescue EOFError
+        end
+        ssl.write("GOT:#{received.bytesize}")
+      ensure
+        ssl.close rescue nil
+      end
+    }
+
+    start_server0(PORT, OpenSSL::SSL::VERIFY_NONE, true,
+                  server_proc: server_proc) do |server, port|
+      sock = TCPSocket.new("127.0.0.1", port)
+      sock.setsockopt(Socket::SOL_SOCKET, Socket::SO_SNDBUF, 2048)
+      ssl = OpenSSL::SSL::SSLSocket.new(sock)
+      ssl.connect
+      ssl.sync_close = true
+
+      remaining = data
+      while remaining.bytesize > 0
+        begin
+          written = ssl.write_nonblock(remaining)
+          remaining = remaining.byteslice(written..-1)
+        rescue IO::WaitWritable
+          IO.select(nil, [ssl])
+          retry
+        end
+      end
+
+      response = ""
+      deadline = Time.now + 30
+      loop do
+        remaining_time = deadline - Time.now
+        break if remaining_time <= 0
+        if IO.select([ssl], nil, nil, [remaining_time, 1].min)
+          begin
+            chunk = ssl.read_nonblock(16384, exception: false)
+            case chunk
+            when :wait_readable then next
+            when nil then break
+            else response << chunk
+            end
+          rescue EOFError
+            break
+          end
+        end
+      end
+
+      assert_match(/^GOT:#{data_size}$/, response,
+        "Server should receive all #{data_size} bytes")
+      ssl.close
+    end
+  end
+
+end