diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 69cf1193b6..3de5af22c5 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,18 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Added support for partial indices to PostgreSQL adapter
+
+    The `add_index` method now supports a `where` option that receives a
+    string with the partial index criteria.
+
+      add_index(:accounts, :code, :where => "active")
+
+      Generates
+
+      CREATE INDEX index_accounts_on_code ON accounts(code) WHERE active
+
+    *Marcelo Silveira*
+
 *   Implemented ActiveRecord::Relation#none method
 
     The `none` method returns a chainable relation with zero records
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index 132ca10f79..ad2e8634eb 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -6,7 +6,7 @@ require 'bigdecimal/util'
 
 module ActiveRecord
   module ConnectionAdapters #:nodoc:
-    class IndexDefinition < Struct.new(:table, :name, :unique, :columns, :lengths, :orders) #:nodoc:
+    class IndexDefinition < Struct.new(:table, :name, :unique, :columns, :lengths, :orders, :where) #:nodoc:
     end
 
     # Abstract representation of a column definition. Instances of this type
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 1f9321edb6..ea6071ea46 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -381,9 +381,16 @@ module ActiveRecord
       #
       # Note: mysql doesn't yet support index order (it accepts the syntax but ignores it)
       #
+      # ====== Creating a partial index
+      #  add_index(:accounts, [:branch_id, :party_id], :unique => true, :where => "active")
+      # generates
+      #  CREATE UNIQUE INDEX index_accounts_on_branch_id_and_party_id ON accounts(branch_id, party_id) WHERE active
+      #
+      # Note: only supported by PostgreSQL
+      #
       def add_index(table_name, column_name, options = {})
-        index_name, index_type, index_columns = add_index_options(table_name, column_name, options)
-        execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{index_columns})"
+        index_name, index_type, index_columns, index_options = add_index_options(table_name, column_name, options)
+        execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{index_columns})#{index_options}"
       end
 
       # Remove the given index from the table.
@@ -581,6 +588,9 @@ module ActiveRecord
           if Hash === options # legacy support, since this param was a string
             index_type = options[:unique] ? "UNIQUE" : ""
             index_name = options[:name].to_s if options.key?(:name)
+            if supports_partial_index?
+              index_options = options[:where] ? " WHERE #{options[:where]}" : ""
+            end
           else
             index_type = options
           end
@@ -593,7 +603,7 @@ module ActiveRecord
           end
           index_columns = quoted_columns_for_index(column_names, options).join(", ")
 
-          [index_name, index_type, index_columns]
+          [index_name, index_type, index_columns, index_options]
         end
 
         def index_name_for_remove(table_name, options = {})
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index edea414db7..dd421b2054 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -142,6 +142,11 @@ module ActiveRecord
         false
       end
 
+      # Does this adapter support partial indices?
+      def supports_partial_index?
+        false
+      end
+
       # Does this adapter support explain? As of this writing sqlite3,
       # mysql2, and postgresql are the only ones that do.
       def supports_explain?
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 194c814e5b..f1940ea15f 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -302,6 +302,10 @@ module ActiveRecord
         true
       end
 
+      def supports_partial_index?
+        true
+      end
+
       class StatementPool < ConnectionAdapters::StatementPool
         def initialize(connection, max)
           super
@@ -885,8 +889,9 @@ module ActiveRecord
           # add info on sort order for columns (only desc order is explicitly specified, asc is the default)
           desc_order_columns = inddef.scan(/(\w+) DESC/).flatten
           orders = desc_order_columns.any? ? Hash[desc_order_columns.map {|order_column| [order_column, :desc]}] : {}
+          where = inddef.scan(/WHERE (.+)$/).flatten[0]
 
-          column_names.empty? ? nil : IndexDefinition.new(table_name, index_name, unique, column_names, [], orders)
+          column_names.empty? ? nil : IndexDefinition.new(table_name, index_name, unique, column_names, [], orders, where)
         end.compact
       end
 
diff --git a/activerecord/lib/active_record/schema_dumper.rb b/activerecord/lib/active_record/schema_dumper.rb
index 2a565b51c6..dcbd165e58 100644
--- a/activerecord/lib/active_record/schema_dumper.rb
+++ b/activerecord/lib/active_record/schema_dumper.rb
@@ -197,6 +197,8 @@ HEADER
             index_orders = (index.orders || {})
             statement_parts << (':order => ' + index.orders.inspect) unless index_orders.empty?
 
+            statement_parts << (':where => ' + index.where.inspect) if index.where
+
             '  ' + statement_parts.join(', ')
           end
 
diff --git a/activerecord/test/cases/adapters/postgresql/active_schema_test.rb b/activerecord/test/cases/adapters/postgresql/active_schema_test.rb
index e4746d4aa3..447d729e52 100644
--- a/activerecord/test/cases/adapters/postgresql/active_schema_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/active_schema_test.rb
@@ -21,6 +21,18 @@ class PostgresqlActiveSchemaTest < ActiveRecord::TestCase
     assert_equal %(CREATE DATABASE "aimonetti" ENCODING = 'latin1'), create_database(:aimonetti, :encoding => :latin1)
   end
 
+  def test_add_index
+    # add_index calls index_name_exists? which can't work since execute is stubbed
+    ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.send(:define_method, :index_name_exists?) do |*|
+      false
+    end
+
+    expected = %(CREATE UNIQUE INDEX "index_people_on_last_name" ON "people" ("last_name") WHERE state = 'active')
+    assert_equal expected, add_index(:people, :last_name, :unique => true, :where => "state = 'active'")
+
+    ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.send(:remove_method, :index_name_exists?)
+  end
+
   private
     def method_missing(method_symbol, *arguments)
       ActiveRecord::Base.connection.send(method_symbol, *arguments)
diff --git a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
index d57794daf8..898d28456b 100644
--- a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
@@ -179,6 +179,12 @@ module ActiveRecord
         assert_equal Arel.sql('$2'), bind
       end
 
+      def test_partial_index
+        @connection.add_index 'ex', %w{ id number }, :name => 'partial', :where => "number > 100"
+        index = @connection.indexes('ex').find { |idx| idx.name == 'partial' }
+        assert_equal "(number > 100)", index.where
+      end
+
       private
       def insert(ctx, data)
         binds   = data.map { |name, value|
diff --git a/activerecord/test/cases/migration/index_test.rb b/activerecord/test/cases/migration/index_test.rb
index 89cf0f5e93..dd9492924c 100644
--- a/activerecord/test/cases/migration/index_test.rb
+++ b/activerecord/test/cases/migration/index_test.rb
@@ -171,6 +171,15 @@ module ActiveRecord
         end
       end
 
+      def test_add_partial_index
+        skip 'only on pg' unless current_adapter?(:PostgreSQLAdapter)
+
+        connection.add_index("testings", "last_name", :where => "first_name = 'john doe'")
+        assert connection.index_exists?("testings", "last_name")
+
+        connection.remove_index("testings", "last_name")
+        assert !connection.index_exists?("testings", "last_name")
+      end
     end
   end
 end
diff --git a/activerecord/test/cases/schema_dumper_test.rb b/activerecord/test/cases/schema_dumper_test.rb
index abeb56fd3f..c79382a984 100644
--- a/activerecord/test/cases/schema_dumper_test.rb
+++ b/activerecord/test/cases/schema_dumper_test.rb
@@ -185,6 +185,15 @@ class SchemaDumperTest < ActiveRecord::TestCase
     assert_equal 'add_index "companies", ["firm_id", "type", "rating", "ruby_type"], :name => "company_index"', index_definition
   end
 
+  def test_schema_dumps_partial_indices
+    index_definition = standard_dump.split(/\n/).grep(/add_index.*company_partial_index/).first.strip
+    if current_adapter?(:PostgreSQLAdapter)
+      assert_equal 'add_index "companies", ["firm_id", "type"], :name => "company_partial_index", :where => "(rating > 10)"', index_definition
+    else
+      assert_equal 'add_index "companies", ["firm_id", "type"], :name => "company_partial_index"', index_definition
+    end
+  end
+
   def test_schema_dump_should_honor_nonstandard_primary_keys
     output = standard_dump
     match = output.match(%r{create_table "movies"(.*)do})
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index da0c4cecdd..d473680fdc 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -175,6 +175,7 @@ ActiveRecord::Schema.define do
   end
 
   add_index :companies, [:firm_id, :type, :rating, :ruby_type], :name => "company_index"
+  add_index :companies, [:firm_id, :type], :name => "company_partial_index", :where => "rating > 10"
 
   create_table :computers, :force => true do |t|
     t.integer :developer, :null => false