diff --git a/lib/rdoc/markup/to_html.rb b/lib/rdoc/markup/to_html.rb
index 6477f98910..5897fa0cbf 100644
--- a/lib/rdoc/markup/to_html.rb
+++ b/lib/rdoc/markup/to_html.rb
@@ -154,6 +154,13 @@ def initialize(pipe: false, output_decoration: true)
 
       UNSAFE_URL_SCHEME_REGEXP = /\A(?:javascript|vbscript|data|file):/i
 
+      ##
+      # Splits the part of an rdoc-image link after "rdoc-image:" into the
+      # image URL and the alt text.  A port number after the host is part of
+      # the URL.
+
+      IMAGE_URL_AND_ALT_REGEXP = %r{\A((?:[A-Za-z][A-Za-z0-9+.\-]*://[^/:?#]*(?::\d+(?=[/?#:]|\z))?)?[^:]*)(?::(.*))?\z}m
+
       # :nodoc:
       URL_CHARACTERS_REGEXP_STR = /[A-Za-z0-9\-._~:\/\?#\[\]@!$&'\(\)*+,;%=]/.source
 
@@ -203,7 +210,8 @@ def handle_RDOCLINK(url) # :nodoc:
           # Split the string after "rdoc-image:" into url and alt.
           #   "path/to/image.jpg:alt text" => ["path/to/image.jpg", "alt text"]
           #   "http://example.com/path/to/image.jpg:alt text" => ["http://example.com/path/to/image.jpg", "alt text"]
-          url, alt = $'.split(/:(?!\/)/, 2)
+          #   "http://example.com:8080/image.jpg:alt text" => ["http://example.com:8080/image.jpg", "alt text"]
+          url, alt = $'.match(IMAGE_URL_AND_ALT_REGEXP).captures
           if alt && !alt.empty?
             %[<img src="#{CGI.escapeHTML(url)}" alt="#{CGI.escapeHTML(alt)}">]
           else
diff --git a/test/rdoc/markup/to_html_test.rb b/test/rdoc/markup/to_html_test.rb
index 6d0107602c..c1c3e220f6 100644
--- a/test/rdoc/markup/to_html_test.rb
+++ b/test/rdoc/markup/to_html_test.rb
@@ -903,6 +903,22 @@ def test_convert_TIDYLINK_image_external
     assert_equal expected, result
   end
 
+  def test_convert_TIDYLINK_image_external_with_port
+    result =
+      @to.convert '{rdoc-image:http://example.com:8080/path/to/image.jpg:alt text}[http://example.com]'
+
+    expected =
+      "\n<p><a href=\"http://example.com\"><img src=\"http://example.com:8080/path/to/image.jpg\" alt=\"alt text\"></a></p>\n"
+
+    assert_equal expected, result
+  end
+
+  def test_convert_RDOCLINK_image_external_with_port_without_alt
+    result = @to.convert 'rdoc-image:http://localhost:8080/image.png'
+
+    assert_equal "\n<p><img src=\"http://localhost:8080/image.png\"></p>\n", result
+  end
+
   def test_convert_TIDYLINK_image_external_with_alt
     result =
       @to.convert '{rdoc-image:http://example.com/path/to/image.jpg:alt text}[http://example.com]'