diff --git a/activesupport/lib/active_support/values/time_zone.rb b/activesupport/lib/active_support/values/time_zone.rb
index b9766a236a..03b324764b 100644
--- a/activesupport/lib/active_support/values/time_zone.rb
+++ b/activesupport/lib/active_support/values/time_zone.rb
@@ -310,13 +310,15 @@ module ActiveSupport
     end
 
     def tzinfo
-      @tzinfo ||= find_tzinfo
+      @tzinfo ||= TimeZone.find_tzinfo(name)
     end
 
     # TODO: Preload instead of lazy load for thread safety
-    def find_tzinfo
+    def self.find_tzinfo(name)
       require 'tzinfo' unless defined?(::TZInfo)
-      ::TZInfo::Timezone.get(MAPPING[name])
+      ::TZInfo::Timezone.get(MAPPING[name] || name)
+    rescue TZInfo::InvalidTimezoneIdentifier
+      nil
     end
 
     unless const_defined?(:ZONES)
@@ -330,7 +332,6 @@ module ActiveSupport
       end
       ZONES.sort!
       ZONES.freeze
-      ZONES_MAP.freeze
 
       US_ZONES = ZONES.find_all { |z| z.name =~ /US|Arizona|Indiana|Hawaii|Alaska/ }
       US_ZONES.freeze
@@ -361,7 +362,7 @@ module ActiveSupport
       def [](arg)
         case arg
           when String
-            ZONES_MAP[arg]
+            ZONES_MAP[arg] ||= lookup(arg)
           when Numeric, ActiveSupport::Duration
             arg *= 3600 if arg.abs <= 13
             all.find { |z| z.utc_offset == arg.to_i }
@@ -375,6 +376,12 @@ module ActiveSupport
       def us_zones
         US_ZONES
       end
+
+      private
+
+        def lookup(name)
+          (tzinfo = find_tzinfo(name)) && create(tzinfo.name.freeze)
+        end
     end
   end
 end
diff --git a/activesupport/test/time_zone_test.rb b/activesupport/test/time_zone_test.rb
index ce43c6507d..68027f7c94 100644
--- a/activesupport/test/time_zone_test.rb
+++ b/activesupport/test/time_zone_test.rb
@@ -75,6 +75,14 @@ class TimeZoneTest < Test::Unit::TestCase
     end
   end
 
+  def test_unknown_timezones_delegation_to_tzinfo
+    zone = ActiveSupport::TimeZone['America/Montevideo']
+    assert_equal ActiveSupport::TimeZone, zone.class
+    assert_equal zone.object_id, ActiveSupport::TimeZone['America/Montevideo'].object_id
+    assert_equal Time.utc(2010, 1, 31, 22), zone.utc_to_local(Time.utc(2010, 2)) # daylight saving offset -0200
+    assert_equal Time.utc(2010, 3, 31, 21), zone.utc_to_local(Time.utc(2010, 4)) # standard offset -0300
+  end
+
   def test_today
     Time.stubs(:now).returns(Time.utc(2000, 1, 1, 4, 59, 59)) # 1 sec before midnight Jan 1 EST
     assert_equal Date.new(1999, 12, 31), ActiveSupport::TimeZone['Eastern Time (US & Canada)'].today