diff --git a/lib/rubygems/installer.rb b/lib/rubygems/installer.rb
index 0534d7c36b17c8..162a50fdb76813 100644
--- a/lib/rubygems/installer.rb
+++ b/lib/rubygems/installer.rb
@@ -343,6 +343,8 @@ def install
 
     Gem::Specification.add_spec(spec)
 
+    load_plugin
+
     run_post_install_hooks
 
     spec
@@ -1006,4 +1008,17 @@ def bash_prolog_script
       ""
     end
   end
+
+  def load_plugin
+    specs = Gem::Specification.find_all_by_name(spec.name)
+    # If old version already exists, this plugin isn't loaded
+    # immediately. It's for avoiding a case that multiple versions
+    # are loaded at the same time.
+    return unless specs.size == 1
+
+    plugin_files = spec.plugins.map do |plugin|
+      File.join(@plugins_dir, "#{spec.name}_plugin#{File.extname(plugin)}")
+    end
+    Gem.load_plugin_files(plugin_files)
+  end
 end
diff --git a/test/rubygems/test_gem.rb b/test/rubygems/test_gem.rb
index 126aad2602a27d..6d36a8bb629fa5 100644
--- a/test/rubygems/test_gem.rb
+++ b/test/rubygems/test_gem.rb
@@ -1449,6 +1449,8 @@ def test_self_vendor_dir_missing
   def test_load_plugins
     plugin_path = File.join "lib", "rubygems_plugin.rb"
 
+    foo1_plugin_path = nil
+    foo2_plugin_path = nil
     Dir.chdir @tempdir do
       FileUtils.mkdir_p "lib"
       File.open plugin_path, "w" do |fp|
@@ -1458,17 +1460,22 @@ def test_load_plugins
       foo1 = util_spec "foo", "1" do |s|
         s.files << plugin_path
       end
+      foo1_plugin_path = File.join(foo1.gem_dir, plugin_path)
 
       install_gem foo1
 
       foo2 = util_spec "foo", "2" do |s|
         s.files << plugin_path
       end
+      foo2_plugin_path = File.join(foo2.gem_dir, plugin_path)
 
       install_gem foo2
     end
 
     Gem::Specification.reset
+    PLUGINS_LOADED.clear
+    $LOADED_FEATURES.delete(foo1_plugin_path)
+    $LOADED_FEATURES.delete(foo2_plugin_path)
 
     gem "foo"
 
diff --git a/test/rubygems/test_gem_commands_pristine_command.rb b/test/rubygems/test_gem_commands_pristine_command.rb
index 51b097dfbda634..ff5e6f166d85b3 100644
--- a/test/rubygems/test_gem_commands_pristine_command.rb
+++ b/test/rubygems/test_gem_commands_pristine_command.rb
@@ -546,7 +546,7 @@ def test_execute_only_plugins
       fp.puts "puts __FILE__"
     end
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |fp|
-      fp.puts "puts __FILE__"
+      fp.puts "# do nothing"
     end
     write_file File.join(@tempdir, "bin", "foo") do |fp|
       fp.puts "#!/usr/bin/ruby"
diff --git a/test/rubygems/test_gem_commands_setup_command.rb b/test/rubygems/test_gem_commands_setup_command.rb
index 41fe4e0045f9fc..cd94f61247cfb2 100644
--- a/test/rubygems/test_gem_commands_setup_command.rb
+++ b/test/rubygems/test_gem_commands_setup_command.rb
@@ -431,7 +431,7 @@ def gem_install_with_plugin(name)
       s.files = %W[lib/rubygems_plugin.rb]
     end
     write_file File.join @tempdir, "lib", "rubygems_plugin.rb" do |f|
-      f.puts "require '#{gem.plugins.first}'"
+      f.puts "# do nothing"
     end
     install_gem gem
 
diff --git a/test/rubygems/test_gem_installer.rb b/test/rubygems/test_gem_installer.rb
index 30113c629bce28..3a4566ce8ceb43 100644
--- a/test/rubygems/test_gem_installer.rb
+++ b/test/rubygems/test_gem_installer.rb
@@ -769,7 +769,7 @@ def test_generate_bin_with_dangling_symlink
   def test_generate_plugins
     installer = util_setup_installer do |spec|
       write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-        io.write "puts __FILE__"
+        io.write "# do nothing"
       end
 
       spec.files += %w[lib/rubygems_plugin.rb]
@@ -856,11 +856,59 @@ def test_generate_plugins_with_build_root
     refute_includes File.read(build_root_path), build_root
   end
 
+  class << self
+    attr_accessor :plugin_loaded
+    attr_accessor :post_install_is_called
+  end
+
+  def test_use_plugin_immediately
+    self.class.plugin_loaded = false
+    self.class.post_install_is_called = false
+    spec_version = nil
+    plugin_path = nil
+    installer = util_setup_installer do |spec|
+      spec_version = spec.version
+      plugin_path = File.join("lib", "rubygems_plugin.rb")
+      write_file File.join(@tempdir, plugin_path) do |io|
+        io.write <<-PLUGIN
+#{self.class}.plugin_loaded = true
+Gem.post_install do
+  #{self.class}.post_install_is_called = true
+end
+        PLUGIN
+      end
+      spec.files += [plugin_path]
+      plugin_path = File.join(spec.gem_dir, plugin_path)
+    end
+    build_rake_in do
+      installer.install
+    end
+    assert self.class.plugin_loaded, "plugin is not loaded"
+    assert self.class.post_install_is_called,
+           "post install hook registered by plugin is not called"
+
+    self.class.plugin_loaded = false
+    $LOADED_FEATURES.delete(plugin_path)
+    installer_new = util_setup_installer do |spec_new|
+      spec_new.version = spec_version.version.succ
+      plugin_path = File.join("lib", "rubygems_plugin.rb")
+      write_file File.join(@tempdir, plugin_path) do |io|
+        io.write "#{self.class}.plugin_loaded = true"
+      end
+      spec_new.files += [plugin_path]
+    end
+    build_rake_in do
+      installer_new.install
+    end
+    assert !self.class.plugin_loaded,
+           "plugin is loaded even when old version is already loaded"
+  end
+
   def test_keeps_plugins_up_to_date
     # NOTE: version a-2 is already installed by setup hooks
 
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-      io.write "puts __FILE__"
+      io.write "# do nothing"
     end
 
     build_rake_in do
diff --git a/test/rubygems/test_gem_uninstaller.rb b/test/rubygems/test_gem_uninstaller.rb
index dd296b080734b9..dfa01768bc349b 100644
--- a/test/rubygems/test_gem_uninstaller.rb
+++ b/test/rubygems/test_gem_uninstaller.rb
@@ -173,7 +173,7 @@ def test_remove_symlinked_gem_home
 
   def test_remove_plugins
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-      io.write "puts __FILE__"
+      io.write "# do nothing"
     end
 
     @spec.files += %w[lib/rubygems_plugin.rb]
@@ -190,7 +190,7 @@ def test_remove_plugins
 
   def test_remove_plugins_with_install_dir
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-      io.write "puts __FILE__"
+      io.write "# do nothing"
     end
 
     @spec.files += %w[lib/rubygems_plugin.rb]
@@ -208,7 +208,7 @@ def test_remove_plugins_with_install_dir
 
   def test_regenerate_plugins_for
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-      io.write "puts __FILE__"
+      io.write "# do nothing"
     end
 
     @spec.files += %w[lib/rubygems_plugin.rb]
@@ -635,7 +635,7 @@ def test_uninstall_no_permission
 
   def test_uninstall_keeps_plugins_up_to_date
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-      io.write "puts __FILE__"
+      io.write "# do nothing"
     end
 
     plugin_path = File.join Gem.plugindir, "a_plugin.rb"