diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6cd9df1..82199cf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Changes
 
+* Use `OpenSSL::X509::DEFAULT_CERT_FILE` by default on MRI. `SSLConfig#cacert_file=` can select another CA file, including the bundled certificates via `SSLConfig::BUNDLED_CACERT_FILE` ([#480](https://github.com/nahi/httpclient/issues/480)).
+
 ## Change Log
 
 ### Changes in 2.10.0
diff --git a/README.md b/README.md
index 80e695b..60d7989 100644
--- a/README.md
+++ b/README.md
@@ -72,6 +72,26 @@ You can install httpclient via rubygems: `gem install httpclient`
 See [HTTPClient](http://www.rubydoc.info/gems/httpclient/frames) for documentation.
 You can also check sample/howto.rb how to use APIs.
 
+### TLS trust store
+
+On MRI, HTTPClient uses `OpenSSL::X509::DEFAULT_CERT_FILE` by default. To
+preserve compatibility with previous releases' default CA selection, select the
+bundled Mozilla CA certificates explicitly:
+
+```ruby
+client = HTTPClient.new
+client.ssl_config.cacert_file = HTTPClient::SSLConfig::BUNDLED_CACERT_FILE
+```
+
+Assigning `cacert_file` loads that file into a new certificate store, replacing
+the current store and resetting existing sessions. You can assign it after a
+connection has been made. `load_trust_ca` adds the selected file, if any, to the
+current store and resets existing sessions. On JRuby, CA selection defaults to
+the JVM trust store and can be overridden with `SSL_CERT_FILE`; assigning
+`cacert_file` explicitly selects a certificate file. Use
+`client.ssl_config.set_default_paths` on MRI to load OpenSSL's configured
+default certificate file and directory.
+
 ### TLS certificate restrictions
 
 Blocked public keys are identified by their SPKI SHA-256 hashes in
@@ -91,11 +111,10 @@ context. With peer verification enabled, its return value controls certificate
 verification. The caller is responsible for the custom callback's validation
 policy. Hostname verification is unchanged.
 
-Note: MRI uses HTTPClient's bundled `cacert.pem` by default, while JRuby uses
-the JVM's CA certificates and certificate validation. Secure operation on JRuby
-depends on keeping the JVM and its CA certificates up to date. HTTPClient's
-additional check may not detect blocked intermediates found only in a local
-trust store or obtained through JVM issuer retrieval (such as AIA when enabled).
+On JRuby, HTTPClient uses the JVM's CA certificates and validation by default.
+Keep the JVM and its CA certificates up to date. The additional blocked-key
+check may not detect intermediates found only in a local trust store or obtained
+through JVM issuer retrieval (such as AIA when enabled).
 
 Remove unused certificates containing blocked public keys from the server's
 certificate chain configuration. If a certificate containing a blocked public key
diff --git a/lib/httpclient/ssl_config.rb b/lib/httpclient/ssl_config.rb
index 2994442..5fd5589 100644
--- a/lib/httpclient/ssl_config.rb
+++ b/lib/httpclient/ssl_config.rb
@@ -20,23 +20,20 @@ class HTTPClient
   #
   # == Trust Anchor Control
   #
-  # SSLConfig loads 'httpclient/cacert.pem' as a trust anchor
-  # (trusted certificate(s)) with add_trust_ca in initialization time.
-  # This means that HTTPClient instance trusts some CA certificates by default,
-  # like Web browsers.  'httpclient/cacert.pem' is downloaded from curl web
-  # site by the author and included in released package.
+  # On MRI, HTTPClient uses OpenSSL::X509::DEFAULT_CERT_FILE by default. To
+  # preserve compatibility with previous releases' default CA selection,
+  # explicitly select the bundled Mozilla CA certificates:
   #
-  # On JRuby, HTTPClient uses Java runtime's trusted CA certificates, not
-  # cacert.pem by default. You can load cacert.pem by calling
-  # SSLConfig#load_trust_ca manually like:
+  #   client = HTTPClient.new
+  #   client.ssl_config.cacert_file = HTTPClient::SSLConfig::BUNDLED_CACERT_FILE
   #
-  #   HTTPClient.new { self.ssl_config.load_trust_ca }.get("https://...")
+  # Assigning cacert_file loads the selected file into a new certificate store,
+  # replacing the current store and resetting existing sessions. It can be
+  # changed after connections have been made. load_trust_ca loads the selected
+  # file, if any, into the current store, adding to its existing certificates.
   #
-  # Secure operation on JRuby depends on keeping the JVM and its CA certificates
-  # up to date.
-  #
-  # You may want to change trust anchor by yourself.  Call clear_cert_store
-  # then add_trust_ca for that purpose.
+  # On JRuby, CA selection defaults to the JVM's trust store and can be
+  # overridden with SSL_CERT_FILE. Assign cacert_file to select a file directly.
   #
   # == TLS certificate restrictions
   #
@@ -116,6 +113,8 @@ def attr_config(symbol)
 
 
     CIPHERS_DEFAULT = "ALL:!aNULL:!eNULL:!SSLv2" # OpenSSL >1.0.0 default
+    # Bundled Mozilla CA certificates, available for explicit selection.
+    BUNDLED_CACERT_FILE = File.join(File.dirname(__FILE__), 'cacert.pem').freeze
 
     # Public keys whose SPKI SHA-256 hashes are listed below are considered blocked.
     # The list covers eight eMudhra intermediate CA public keys whose corresponding
@@ -197,9 +196,12 @@ def self.blocked_public_key?(public_key_der)
     # See ciphers(1) man in OpenSSL for more detail.
     attr_config :ciphers
 
-    # OpenSSL::X509::X509::Store used for verification.  You can reset the
-    # store with clear_cert_store and set the new store with cert_store=.
+    # OpenSSL::X509::Store used for verification. You can reset the store with
+    # clear_cert_store and set the new store with cert_store=.
     attr_reader :cert_store # don't use if you don't know what it is.
+    # Selected CA certificate file. Defaults to OpenSSL::X509::DEFAULT_CERT_FILE
+    # on MRI and nil on JRuby, where CA selection is left to the runtime.
+    attr_reader :cacert_file
 
     # For server side configuration.  Ignore this.
     attr_config :client_ca # :nodoc:
@@ -214,6 +216,8 @@ def cert_store_items; @cert_store._httpclient_cert_store_items; end
     def initialize(client)
       return unless SSLEnabled
       @client = client
+      # nil leaves JRuby's default CA selection (JVM store or SSL_CERT_FILE) intact.
+      @cacert_file = defined?(JRUBY_VERSION) ? nil : X509::DEFAULT_CERT_FILE
       @cert_store = X509::Store.new
       @cert_store_crl_items = []
       @client_cert = @client_key = @client_key_pass = @client_ca = nil
@@ -309,6 +313,17 @@ def add_trust_ca(trust_ca_file_or_hashed_dir)
     end
     alias set_trust_ca add_trust_ca
 
+    # Selects a CA certificate file. Replaces the current certificate store
+    # with a new store loaded from file and resets existing sessions. If loading
+    # fails, the current store and selected file are unchanged.
+    def cacert_file=(file)
+      store = X509::Store.new
+      store._httpclient_cert_store_items.clear if defined?(JRUBY_VERSION)
+      add_trust_ca_to_store(store, file)
+      @cacert_file = file
+      self.cert_store = store
+    end
+
     def add_trust_ca_to_store(cert_store, trust_ca_file_or_hashed_dir)
       if FileTest.directory?(trust_ca_file_or_hashed_dir)
         cert_store.add_path(trust_ca_file_or_hashed_dir)
@@ -317,8 +332,9 @@ def add_trust_ca_to_store(cert_store, trust_ca_file_or_hashed_dir)
       end
     end
 
-    # Loads default trust anchors.
-    # Calling this method resets all existing sessions.
+    # Loads the selected cacert_file into the current certificate store.
+    # Existing certificates in the store are kept. Calling this method resets
+    # all existing sessions.
     def load_trust_ca
       load_cacerts(@cert_store)
       change_notify
@@ -521,10 +537,8 @@ def change_notify
       nil
     end
 
-    # Use 2048 bit certs trust anchor
     def load_cacerts(cert_store)
-      file = File.join(File.dirname(__FILE__), 'cacert.pem')
-      add_trust_ca_to_store(cert_store, file)
+      add_trust_ca_to_store(cert_store, @cacert_file) unless @cacert_file.nil?
     end
   end
 
diff --git a/test/test_ssl.rb b/test/test_ssl.rb
index c837274..1f615a2 100644
--- a/test/test_ssl.rb
+++ b/test/test_ssl.rb
@@ -106,6 +106,8 @@ def test_blocked_peer_certificate_before_request
 
   def test_blocked_intermediate_with_default_paths
     escape_env do
+      # Nix's OpenSSL can prioritize this CA bundle path over SSL_CERT_FILE.
+      ENV.delete('NIX_SSL_CERT_FILE')
       ENV['SSL_CERT_FILE'] = path('ca-chain.pem')
       @client.ssl_config.set_default_paths
       @server.ssl_context.extra_chain_cert = [cert('subca.cert')]
@@ -373,6 +375,8 @@ def test_set_default_paths
       @client.get(@url)
     end
     escape_env do
+      # Nix's OpenSSL can prioritize this CA bundle path over SSL_CERT_FILE.
+      ENV.delete('NIX_SSL_CERT_FILE')
       ENV['SSL_CERT_FILE'] = File.join(DIR, 'ca-chain.pem')
       @client.ssl_config.set_default_paths
       @client.get(@url)
diff --git a/test/test_ssl_config.rb b/test/test_ssl_config.rb
index 67b82d2..c013f5a 100644
--- a/test/test_ssl_config.rb
+++ b/test/test_ssl_config.rb
@@ -49,3 +49,88 @@ def test_blocked_cross_signed_keys_without_der_export
     end
   end
 end
+
+class TestCACertFile < Test::Unit::TestCase
+  def setup
+    @client = HTTPClient.new
+    @config = @client.ssl_config
+    @ca_file = File.join(__dir__, 'ca.cert')
+  end
+
+  def teardown
+    @client.reset_all
+  end
+
+  if defined?(JRUBY_VERSION)
+    def test_load_trust_ca_keeps_default_selection_when_no_file_is_selected
+      default_items = @config.cert_store_items.dup
+      @config.load_trust_ca
+
+      assert_nil(@config.cacert_file)
+      assert_equal(default_items, @config.cert_store_items)
+    end
+  end
+
+  def test_cacert_file_replaces_store
+    @config.add_trust_ca(File.join(__dir__, 'ca-chain.pem'))
+    original_store = @config.cert_store
+    @config.cacert_file = @ca_file
+
+    assert_not_same(original_store, @config.cert_store)
+    assert_equal(@ca_file, @config.cacert_file)
+    assert_equal([@ca_file], @config.cert_store_items) if defined?(JRUBY_VERSION)
+  end
+
+  def test_load_trust_ca_uses_selected_file
+    @config.cacert_file = @ca_file
+    @config.clear_cert_store
+    @config.load_trust_ca
+
+    certificate = OpenSSL::X509::Certificate.new(File.read(@ca_file))
+    @config.cert_store.time = certificate.not_before + 1
+    assert(@config.cert_store.verify(certificate), @config.cert_store.error_string)
+  end
+end
+
+# JRuby uses its JVM trust manager instead of SSLConfig#set_context.
+unless defined?(JRUBY_VERSION)
+  class TestOpenSSLTrustStore < Test::Unit::TestCase
+    def setup
+      @client = HTTPClient.new
+      @config = @client.ssl_config
+      @loaded_files = []
+      loaded_files = @loaded_files
+      @config.define_singleton_method(:add_trust_ca_to_store) do |_store, file|
+        loaded_files << file
+      end
+    end
+
+    def teardown
+      @client.reset_all if @client
+    end
+
+    def test_default_cert_file_is_loaded_once
+      configure_context
+      configure_context
+      assert_equal([OpenSSL::X509::DEFAULT_CERT_FILE], @loaded_files)
+    end
+
+    def test_bundled_ca_can_be_selected_with_cacert_file
+      @config.cacert_file = bundled_ca_file
+      configure_context
+      assert_equal([bundled_ca_file], @loaded_files)
+    end
+
+    private
+
+    def configure_context
+      context = OpenSSL::SSL::SSLContext.new
+      @config.set_context(context)
+      assert_same(@config.cert_store, context.cert_store)
+    end
+
+    def bundled_ca_file
+      HTTPClient::SSLConfig::BUNDLED_CACERT_FILE
+    end
+  end
+end