diff --git a/lib/validated_object.rb b/lib/validated_object.rb
index 4705c89..611715d 100644
--- a/lib/validated_object.rb
+++ b/lib/validated_object.rb
@@ -54,6 +54,21 @@ class Base
     class Boolean
     end
 
+    # A private class definition for union types.
+    # Stores multiple allowed types for validation.
+    # Created via ValidatedObject::Base.union(*types)
+    #
+    # @example
+    #   validates :id, type: union(String, Integer)
+    #   validates :data, type: union(Hash, [Hash])
+    class Union
+      attr_reader :types
+
+      def initialize(*types)
+        @types = types
+      end
+    end
+
     # Instantiate and validate a new object.
     # @example
     #   maru = Dog.new(birthday: Date.today, name: 'Maru')
@@ -99,6 +114,14 @@ def validate_each(record, attribute, value)
         validation_options = options
         expected_class = validation_options[:with]
 
+        # Support union types
+        if expected_class.is_a?(Union)
+          return if validate_union_type(record, attribute, value, expected_class, validation_options)
+
+          save_union_error(record, attribute, value, expected_class, validation_options)
+          return
+        end
+
         # Support type: Array, element_type: ElementType
         if expected_class == Array && validation_options[:element_type]
           return save_error(record, attribute, value, validation_options) unless value.is_a?(Array)
@@ -134,6 +157,50 @@ def save_error(record, attribute, value, validation_options)
         record.errors.add attribute,
                           validation_options[:message] || "is a #{value.class}, not a #{validation_options[:with]}"
       end
+
+      def validate_union_type(_record, _attribute, value, union, _validation_options)
+        union.types.any? do |type_spec|
+          if type_spec.is_a?(Array) && type_spec.length == 1
+            # Handle [ElementType] syntax within union
+            validate_array_element_type(value, type_spec[0])
+          elsif type_spec.is_a?(Class) || type_spec == Boolean
+            # Handle class types (String, Integer, etc.) and pseudo-boolean
+            pseudo_boolean?(type_spec, value) || expected_class?(type_spec, value)
+          else
+            # Handle literal values (symbols, strings, numbers, etc.)
+            value == type_spec
+          end
+        end
+      end
+
+      def validate_array_element_type(value, element_type)
+        return false unless value.is_a?(Array)
+
+        value.all? { |el| el.is_a?(element_type) }
+      end
+
+      def save_union_error(record, attribute, value, union, validation_options)
+        return if validation_options[:message]
+
+        type_names = union.types.map do |type_spec|
+          if type_spec.is_a?(Array) && type_spec.length == 1
+            "Array of #{type_spec[0]}"
+          elsif type_spec.is_a?(Class) || type_spec == Boolean
+            type_spec.to_s
+          else
+            # For literal values like :active, show them as-is
+            type_spec.inspect
+          end
+        end
+
+        message = if type_names.length == 1
+                    "is a #{value.class}, not one of #{type_names.first}"
+                  else
+                    "is a #{value.class}, not one of #{type_names.join(', ')}"
+                  end
+
+        record.errors.add attribute, message
+      end
     end
 
     # Register the TypeValidator with ActiveModel so `type:` validation option works
@@ -146,6 +213,16 @@ def self.validated(*args, **kwargs, &block)
       validates(*args, **kwargs, &block)
     end
 
+    # Create a union type specification for validation
+    # @param types [Array] The types that are allowed
+    # @return [Union] A union type specification
+    # @example
+    #   validates :id, type: union(String, Integer)
+    #   validates :data, type: union(Hash, [Hash])
+    def self.union(*types)
+      Union.new(*types)
+    end
+
     private
 
     def set_instance_variables(from_hash:)
diff --git a/lib/validated_object/simplified_api.rb b/lib/validated_object/simplified_api.rb
index 41560d7..12ac72d 100644
--- a/lib/validated_object/simplified_api.rb
+++ b/lib/validated_object/simplified_api.rb
@@ -1,8 +1,10 @@
+# frozen_string_literal: true
+
 require 'active_support/concern'
 
-# Enable a simplified API for the common case of
-# read-only ValidatedObjects.
 module ValidatedObject
+  # Enable a simplified API for the common case of
+  # read-only ValidatedObjects.
   module SimplifiedApi
     extend ActiveSupport::Concern
 
@@ -19,7 +21,6 @@ def validated(*args, **kwargs, &block)
         validates(*args, **kwargs, &block)
       end
 
-      # Alias for validated_attr for compatibility with test usage.
       def validates_attr(attribute, *options, **kwargs)
         attr_reader attribute
 
@@ -27,16 +28,23 @@ def validates_attr(attribute, *options, **kwargs)
           type_val = kwargs.delete(:type)
           element_type = kwargs.delete(:element_type)
 
+          # Handle Union types - pass them through directly
+          if type_val.is_a?(ValidatedObject::Base::Union)
+            opts = { type: { with: type_val } }
+            validates attribute, opts.merge(kwargs)
           # Parse Array[ElementType] syntax
-          if type_val.is_a?(Array) && type_val.length == 1 && type_val[0].is_a?(Class)
+          elsif type_val.is_a?(Array) && type_val.length == 1 && type_val[0].is_a?(Class)
             # This handles Array[Comment] syntax
             element_type = type_val[0]
             type_val = Array
+            opts = { type: { with: type_val } }
+            opts[:type][:element_type] = element_type if element_type
+            validates attribute, opts.merge(kwargs)
+          else
+            opts = { type: { with: type_val } }
+            opts[:type][:element_type] = element_type if element_type
+            validates attribute, opts.merge(kwargs)
           end
-
-          opts = { type: { with: type_val } }
-          opts[:type][:element_type] = element_type if element_type
-          validates attribute, opts.merge(kwargs)
         else
           validates attribute, *options, **kwargs
         end
diff --git a/lib/validated_object/version.rb b/lib/validated_object/version.rb
index 788837c..809d346 100644
--- a/lib/validated_object/version.rb
+++ b/lib/validated_object/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module ValidatedObject
-  VERSION = '2.3.3'
+  VERSION = '2.3.4'
 end
diff --git a/spec/union_types_spec.rb b/spec/union_types_spec.rb
new file mode 100644
index 0000000..19b4594
--- /dev/null
+++ b/spec/union_types_spec.rb
@@ -0,0 +1,93 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'validated_object'
+
+#
+# I needed to create actual classes for these specs to work.
+# Therefore I namespaced them with Spec.
+#
+class SpecUnionComment; end
+
+class SpecMultiType < ValidatedObject::Base
+  attr_accessor :id, :status, :data
+
+  validates :id, type: union(String, Integer)
+  validates :status, type: union(:active, :inactive, :pending)
+  validates :data, type: union(Hash, [Hash]), allow_nil: true
+end
+
+class SpecUnionAttr < ValidatedObject::Base
+  validates_attr :mixed, type: union(String, Integer, [String])
+end
+
+class SpecComplexUnion < ValidatedObject::Base
+  attr_accessor :flexible
+
+  validates :flexible, type: union(String, Integer, [String], [Hash])
+end
+
+class SpecSingleUnion < ValidatedObject::Base
+  attr_accessor :name
+
+  validates :name, type: union(String)
+end
+
+describe 'ValidatedObject Union Types' do
+  context 'when using union types' do
+    it 'accepts values matching first union type (String)' do
+      obj = SpecMultiType.new(id: 'abc123', status: :active, data: { key: 'value' })
+      expect(obj).to be_valid
+    end
+
+    it 'accepts values matching second union type (Integer)' do
+      obj = SpecMultiType.new(id: 42, status: :inactive, data: nil)
+      expect(obj).to be_valid
+    end
+
+    it 'accepts values matching array element type in union' do
+      obj = SpecMultiType.new(id: 'test', status: :pending, data: [{ a: 1 }, { b: 2 }])
+      expect(obj).to be_valid
+    end
+
+    it 'rejects values not matching any union type' do
+      expect do
+        SpecMultiType.new(id: 3.14, status: :active, data: {})
+      end.to raise_error(ArgumentError, /is a Float, not one of String, Integer/)
+    end
+
+    it 'rejects invalid symbol values in union' do
+      expect do
+        SpecMultiType.new(id: 'test', status: :invalid, data: {})
+      end.to raise_error(ArgumentError, /is a Symbol.*not one of.*active.*inactive.*pending/)
+    end
+
+    it 'works with validates_attr syntax' do
+      expect(SpecUnionAttr.new(mixed: 'text')).to be_valid
+      expect(SpecUnionAttr.new(mixed: 42)).to be_valid
+      expect(SpecUnionAttr.new(mixed: %w[a b c])).to be_valid
+
+      expect do
+        SpecUnionAttr.new(mixed: 3.14)
+      end.to raise_error(ArgumentError, /is a Float.*not one of.*String.*Integer.*Array of String/)
+    end
+
+    it 'handles complex union with multiple array types' do
+      expect(SpecComplexUnion.new(flexible: 'text')).to be_valid
+      expect(SpecComplexUnion.new(flexible: 123)).to be_valid
+      expect(SpecComplexUnion.new(flexible: %w[a b])).to be_valid
+      expect(SpecComplexUnion.new(flexible: [{ a: 1 }, { b: 2 }])).to be_valid
+
+      expect do
+        SpecComplexUnion.new(flexible: [1, 2, 3])
+      end.to raise_error(ArgumentError, /is a Array, not one of String, Integer, Array of String, Array of Hash/)
+    end
+
+    it 'works with single type union (equivalent to regular type validation)' do
+      expect(SpecSingleUnion.new(name: 'test')).to be_valid
+      expect do
+        SpecSingleUnion.new(name: 123)
+      end.to raise_error(ArgumentError, /is a Integer, not one of String/)
+    end
+  end
+end