diff --git a/lib/active_record/connection_adapters/spanner/database_statements.rb b/lib/active_record/connection_adapters/spanner/database_statements.rb
index 82ac22ee..7bc52a63 100644
--- a/lib/active_record/connection_adapters/spanner/database_statements.rb
+++ b/lib/active_record/connection_adapters/spanner/database_statements.rb
@@ -378,16 +378,18 @@ def to_types_and_params binds
 
         def to_types binds
           binds.enum_for(:each_with_index).to_h do |bind, i|
-            type = :INT64
-            if bind.respond_to? :type
-              type = ActiveRecord::Type::Spanner::SpannerActiveRecordConverter
-                     .convert_active_model_type_to_spanner(bind.type)
-            elsif bind.instance_of? Symbol
-              # This ensures that for example :environment is sent as the string 'environment' to Cloud Spanner.
-              type = :STRING
-            elsif bind.instance_of?(TrueClass) || bind.instance_of?(FalseClass)
-              type = :BOOL
-            end
+            type = if bind.respond_to? :type
+                     ActiveRecord::Type::Spanner::SpannerActiveRecordConverter
+                       .convert_active_model_type_to_spanner(bind.type)
+                   elsif bind.instance_of? Symbol
+                     # This ensures that for example :environment is sent as the string 'environment' to Cloud Spanner.
+                     :STRING
+                   else
+                     # Untyped binds (e.g. from `Arel.sql("name = ?", "abc")`) are bare Ruby values without an
+                     # attached ActiveModel type. Derive the Spanner type from the Ruby class, defaulting to INT64.
+                     ActiveRecord::Type::Spanner::SpannerActiveRecordConverter
+                       .convert_active_model_type_to_spanner(untyped_bind_type(bind)) || :INT64
+                   end
             [
               # Generates binds for named parameters in the format `@p1, @p2, ...`
               "p#{i + 1}", type
@@ -403,8 +405,8 @@ def to_params binds
                      # This ensures that for example :environment is sent as the string 'environment' to Cloud Spanner.
                      :STRING
                    else
-                     # The Cloud Spanner default type is INT64 if no other type is known.
-                     ActiveModel::Type::Integer
+                     # Untyped bind: pick the serializer that matches the type declared in `to_types`.
+                     untyped_bind_type bind
                    end
             bind_value = bind.respond_to?(:value) ? bind.value : bind
             value = ActiveRecord::Type::Spanner::SpannerActiveRecordConverter
@@ -414,6 +416,21 @@ def to_params binds
           end
         end
 
+        # Maps a bare Ruby value (a bind without an attached ActiveModel type) to the ActiveModel type
+        # that should be used to declare and serialize it. Returns nil for values that are not recognized,
+        # which keeps the historical behavior: the bind is declared as INT64 and the value is sent as-is.
+        def untyped_bind_type value
+          case value
+          when ::String then ActiveModel::Type::String.new
+          when true, false then ActiveModel::Type::Boolean.new
+          when ::Float then ActiveModel::Type::Float.new
+          when ::BigDecimal then ActiveModel::Type::Decimal.new
+          # DateTime is a subclass of Date, so it must be matched before Date.
+          when ::Time, ::DateTime then ActiveRecord::Type::Spanner::Time.new
+          when ::Date then ActiveModel::Type::Date.new
+          end
+        end
+
         # An insert/update/delete statement could use mutations in some specific circumstances.
         # This method returns an indication whether a specific operation should use mutations instead of DML
         # based on the operation itself, and the current transaction.
diff --git a/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb b/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb
index 897315eb..4d24cdfa 100644
--- a/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb
+++ b/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb
@@ -697,6 +697,55 @@ def test_find_singer_by_last_performance_as_non_iso_string
       assert_equal timestamp.utc.rfc3339(9), request.params["p1"]
     end
 
+    def test_untyped_binds_from_arel_sql_are_typed_by_ruby_class
+      select_sql = "SELECT `singers`.* FROM `singers` WHERE first_name = @p1 AND active = @p2 AND weight = @p3 " \
+                   "AND balance = @p4 AND last_performance = @p5 AND created_at = @p6 AND birth_date = @p7 AND age = @p8"
+      @mock.put_statement_result select_sql, MockServerTests::create_random_singers_result(1)
+
+      time = ::Time.parse("2021-05-12T10:30:00+02:00")
+      date_time = ::DateTime.new(2021, 5, 12, 10, 30, 0, "+02:00")
+      Singer.where(
+        Arel.sql(
+          "first_name = ? AND active = ? AND weight = ? AND balance = ? AND last_performance = ? " \
+          "AND created_at = ? AND birth_date = ? AND age = ?",
+          "Alice", true, 1.5, BigDecimal("12.34"), time, date_time, ::Date.new(2021, 5, 12), 42
+        )
+      ).to_a
+
+      request = @mock.requests.select {|req| req.is_a?(Google::Cloud::Spanner::V1::ExecuteSqlRequest) && req.sql == select_sql }.first
+      refute_nil request
+      assert_equal :STRING, request.param_types["p1"].code
+      assert_equal "Alice", request.params["p1"]
+      assert_equal :BOOL, request.param_types["p2"].code
+      assert_equal true, request.params["p2"]
+      assert_equal :FLOAT64, request.param_types["p3"].code
+      assert_equal 1.5, request.params["p3"]
+      assert_equal :NUMERIC, request.param_types["p4"].code
+      assert_equal "12.34", request.params["p4"]
+      assert_equal :TIMESTAMP, request.param_types["p5"].code
+      assert_equal "2021-05-12T08:30:00.000000000Z", request.params["p5"]
+      assert_equal :TIMESTAMP, request.param_types["p6"].code
+      assert_equal "2021-05-12T08:30:00.000000000Z", request.params["p6"]
+      assert_equal :DATE, request.param_types["p7"].code
+      assert_equal "2021-05-12", request.params["p7"]
+      assert_equal :INT64, request.param_types["p8"].code
+      assert_equal "42", request.params["p8"]
+    end
+
+    def test_where_with_positional_string_placeholder
+      # Before ActiveRecord 8.1, `where("col = ?", value)` inlines the value into the SQL instead of binding it.
+      skip "Requires Rails version 8.1 or higher" if ActiveRecord.version < Gem::Version.create("8.1.0")
+      select_sql = "SELECT `singers`.* FROM `singers` WHERE (first_name = @p1)"
+      @mock.put_statement_result select_sql, MockServerTests::create_random_singers_result(1)
+
+      Singer.where("first_name = ?", "Alice").to_a
+
+      request = @mock.requests.select {|req| req.is_a?(Google::Cloud::Spanner::V1::ExecuteSqlRequest) && req.sql == select_sql }.first
+      refute_nil request
+      assert_equal :STRING, request.param_types["p1"].code
+      assert_equal "Alice", request.params["p1"]
+    end
+
     def test_create_singer_with_picture
       insert_sql = "INSERT INTO `singers` (`first_name`, `last_name`, `picture`, `id`) VALUES (@p1, @p2, @p3, @p4)"
       @mock.put_statement_result insert_sql, StatementResult.new(1)