diff --git a/lib/rubygems/commands/sources_command.rb b/lib/rubygems/commands/sources_command.rb
index 7e5c2a2465e6..95be9d334b33 100644
--- a/lib/rubygems/commands/sources_command.rb
+++ b/lib/rubygems/commands/sources_command.rb
@@ -50,6 +50,7 @@ def initialize
   end
 
   def add_source(source_uri) # :nodoc:
+    source_uri = add_trailing_slash(source_uri)
     check_rubygems_https source_uri
 
     source = Gem::Source.new source_uri
@@ -76,6 +77,7 @@ def add_source(source_uri) # :nodoc:
   end
 
   def append_source(source_uri) # :nodoc:
+    source_uri = add_trailing_slash(source_uri)
     check_rubygems_https source_uri
 
     source = Gem::Source.new source_uri
@@ -103,6 +105,7 @@ def append_source(source_uri) # :nodoc:
   end
 
   def prepend_source(source_uri) # :nodoc:
+    source_uri = add_trailing_slash(source_uri)
     check_rubygems_https source_uri
 
     source = Gem::Source.new source_uri
@@ -141,6 +144,19 @@ def check_typo_squatting(source)
     end
   end
 
+  def add_trailing_slash(source_uri) # :nodoc:
+    # Ensure the source URI has a trailing slash for proper RFC 2396 path merging
+    # Without a trailing slash, the last path segment is treated as a file and removed
+    # during relative path resolution (e.g., "/blish" + "gems/foo.gem" = "/gems/foo.gem")
+    # With a trailing slash, it's treated as a directory (e.g., "/blish/" + "gems/foo.gem" = "/blish/gems/foo.gem")
+    uri = Gem::URI.parse(source_uri)
+    uri.path = uri.path.gsub(%r{/+$}, "") + "/" if uri.path && !uri.path.empty?
+    uri.to_s
+  rescue Gem::URI::Error
+    # If parsing fails, return the original URI and let later validation handle it
+    source_uri
+  end
+
   def check_rubygems_https(source_uri) # :nodoc:
     uri = Gem::URI source_uri
 
diff --git a/test/rubygems/test_gem_commands_sources_command.rb b/test/rubygems/test_gem_commands_sources_command.rb
index 00eb9239940e..a0f11bf04ccd 100644
--- a/test/rubygems/test_gem_commands_sources_command.rb
+++ b/test/rubygems/test_gem_commands_sources_command.rb
@@ -60,6 +60,82 @@ def test_execute_add
     assert_equal "", @ui.error
   end
 
+  def test_execute_add_without_trailing_slash
+    setup_fake_source('https://rubygems.pkg.github.com/my-org')
+
+    @cmd.handle_options %W[--add https://rubygems.pkg.github.com/my-org]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@gem_repo, 'https://rubygems.pkg.github.com/my-org/'], Gem.sources
+
+    expected = <<-EOF
+https://rubygems.pkg.github.com/my-org/ added to sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
+  def test_execute_add_multiple_trailing_slash
+    setup_fake_source('https://rubygems.pkg.github.com/my-org/')
+
+    @cmd.handle_options %W[--add https://rubygems.pkg.github.com/my-org///]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@gem_repo, 'https://rubygems.pkg.github.com/my-org/'], Gem.sources
+
+    expected = <<-EOF
+https://rubygems.pkg.github.com/my-org/ added to sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
+  def test_execute_append_without_trailing_slash
+    setup_fake_source('https://rubygems.pkg.github.com/my-org')
+
+    @cmd.handle_options %W[--append https://rubygems.pkg.github.com/my-org]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@gem_repo, 'https://rubygems.pkg.github.com/my-org/'], Gem.sources
+
+    expected = <<-EOF
+https://rubygems.pkg.github.com/my-org/ added to sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
+  def test_execute_prepend_without_trailing_slash
+    setup_fake_source('https://rubygems.pkg.github.com/my-org')
+
+    @cmd.handle_options %W[--prepend https://rubygems.pkg.github.com/my-org]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@gem_repo, 'https://rubygems.pkg.github.com/my-org/'], Gem.sources
+
+    expected = <<-EOF
+https://rubygems.pkg.github.com/my-org/ added to sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_append
     setup_fake_source(@new_repo)
 
@@ -583,7 +659,7 @@ def test_execute_add_bad_uri
     assert_equal [@gem_repo], Gem.sources
 
     expected = <<-EOF
-beta-gems.example.com is not a URI
+beta-gems.example.com/ is not a URI
     EOF
 
     assert_equal expected, @ui.output
@@ -602,7 +678,26 @@ def test_execute_append_bad_uri
     assert_equal [@gem_repo], Gem.sources
 
     expected = <<-EOF
-beta-gems.example.com is not a URI
+beta-gems.example.com/ is not a URI
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
+  def test_execute_prepend_bad_uri
+    @cmd.handle_options %w[--prepend beta-gems.example.com]
+
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    assert_equal [@gem_repo], Gem.sources
+
+    expected = <<-EOF
+beta-gems.example.com/ is not a URI
     EOF
 
     assert_equal expected, @ui.output