diff --git a/lib/hexdocs/file_rewriter.ex b/lib/hexdocs/file_rewriter.ex
index 9243492..f5b0ac3 100644
--- a/lib/hexdocs/file_rewriter.ex
+++ b/lib/hexdocs/file_rewriter.ex
@@ -9,11 +9,14 @@ defmodule Hexdocs.FileRewriter do
 
   @noindex_hook ~s|<meta name="robots" content="noindex">|
 
+  @official_domains ~w(hex.pm hexdocs.pm elixir-lang.org erlang.org)
+
   def run(path, content) do
     content
     |> add_elixir_org_link(path)
     |> add_analytics(path)
     |> remove_noindex(path)
+    |> add_nofollow(path)
   end
 
   defp add_elixir_org_link(content, path) do
@@ -42,4 +45,49 @@ defmodule Hexdocs.FileRewriter do
       content
     end
   end
+
+  @a_tag_re ~r/<a\s[^>]*href="https?:\/\/[^"]*"[^>]*>/
+  @href_re ~r/href="(https?:\/\/[^"]*)"/
+
+  defp add_nofollow(content, path) do
+    if String.ends_with?(path, ".html") do
+      Regex.replace(@a_tag_re, content, fn tag ->
+        case Regex.run(@href_re, tag) do
+          [_, href] ->
+            if official_link?(href) do
+              tag
+            else
+              add_rel_nofollow(tag)
+            end
+
+          _ ->
+            tag
+        end
+      end)
+    else
+      content
+    end
+  end
+
+  defp add_rel_nofollow(tag) do
+    if tag =~ ~r/\srel="/ do
+      Regex.replace(~r/\srel="([^"]*)"/, tag, fn _, existing ->
+        if "nofollow" in String.split(existing) do
+          ~s| rel="#{existing}"|
+        else
+          ~s| rel="#{existing} nofollow"|
+        end
+      end)
+    else
+      String.replace(tag, "<a ", ~s|<a rel="nofollow" |)
+    end
+  end
+
+  defp official_link?(href) do
+    uri = URI.parse(href)
+
+    Enum.any?(@official_domains, fn domain ->
+      uri.host == domain or (uri.host && String.ends_with?(uri.host, "." <> domain))
+    end)
+  end
 end
diff --git a/test/hexdocs/file_rewriter_test.exs b/test/hexdocs/file_rewriter_test.exs
index 3c4c5a9..174a050 100644
--- a/test/hexdocs/file_rewriter_test.exs
+++ b/test/hexdocs/file_rewriter_test.exs
@@ -12,8 +12,54 @@ defmodule Hexdocs.FileRewriterTest do
              "index.html",
              ~s|<a href="https://twitter.com/dignifiedquire" target="_blank" title="@dignifiedquire">Friedel Ziegelmayer</a>|
            ) ==
-             ~s|<a href="https://twitter.com/dignifiedquire" target="_blank" title="@dignifiedquire">Friedel Ziegelmayer</a> for the <a href="https://elixir-lang.org" title="Elixir" target="_blank">Elixir programming language</a>|
+             ~s|<a rel="nofollow" href="https://twitter.com/dignifiedquire" target="_blank" title="@dignifiedquire">Friedel Ziegelmayer</a> for the <a href="https://elixir-lang.org" title="Elixir" target="_blank">Elixir programming language</a>|
 
     assert FileRewriter.run("index.html", ~s|<meta name="robots" content="noindex">|) == ""
   end
+
+  describe "add_nofollow" do
+    test "adds rel=nofollow to external links" do
+      assert FileRewriter.run("index.html", ~s|<a href="https://example.com">example</a>|) ==
+               ~s|<a rel="nofollow" href="https://example.com">example</a>|
+    end
+
+    test "appends nofollow to existing rel attribute" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<a href="https://example.com" rel="help">example</a>|
+             ) ==
+               ~s|<a href="https://example.com" rel="help nofollow">example</a>|
+    end
+
+    test "does not duplicate nofollow" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<a href="https://example.com" rel="nofollow">example</a>|
+             ) ==
+               ~s|<a href="https://example.com" rel="nofollow">example</a>|
+    end
+
+    test "does not add nofollow to official ecosystem links" do
+      for url <- [
+            "https://hex.pm/packages/foo",
+            "https://hexdocs.pm/foo",
+            "https://elixir-lang.org",
+            "https://www.erlang.org",
+            "https://preview.hexdocs.pm/foo"
+          ] do
+        input = ~s|<a href="#{url}">link</a>|
+        assert FileRewriter.run("index.html", input) == input
+      end
+    end
+
+    test "does not add nofollow to relative links" do
+      input = ~s|<a href="other.html">link</a>|
+      assert FileRewriter.run("index.html", input) == input
+    end
+
+    test "does not modify non-html files" do
+      input = ~s|<a href="https://example.com">example</a>|
+      assert FileRewriter.run("index.js", input) == input
+    end
+  end
 end