diff --git a/lib/database_consistency.rb b/lib/database_consistency.rb
index a2f485c..4a82d8d 100644
--- a/lib/database_consistency.rb
+++ b/lib/database_consistency.rb
@@ -38,6 +38,7 @@
 require 'database_consistency/writers/simple/enum_values_inconsistent_with_ar_enum'
 require 'database_consistency/writers/simple/enum_values_inconsistent_with_inclusion'
 require 'database_consistency/writers/simple/redundant_case_insensitive_option'
+require 'database_consistency/writers/simple/three_state_boolean'
 require 'database_consistency/writers/simple_writer'
 
 require 'database_consistency/writers/autofix/helpers/migration'
@@ -71,6 +72,7 @@
 require 'database_consistency/checkers/column_checkers/length_constraint_checker'
 require 'database_consistency/checkers/column_checkers/primary_key_type_checker'
 require 'database_consistency/checkers/column_checkers/enum_value_checker'
+require 'database_consistency/checkers/column_checkers/three_state_boolean_checker'
 
 require 'database_consistency/checkers/validator_checkers/validator_checker'
 require 'database_consistency/checkers/validator_checkers/missing_unique_index_checker'
diff --git a/lib/database_consistency/checkers/column_checkers/three_state_boolean_checker.rb b/lib/database_consistency/checkers/column_checkers/three_state_boolean_checker.rb
new file mode 100644
index 0000000..0615f58
--- /dev/null
+++ b/lib/database_consistency/checkers/column_checkers/three_state_boolean_checker.rb
@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+module DatabaseConsistency
+  module Checkers
+    # This class checks missing NOT NULL constraint for boolean columns
+    class ThreeStateBooleanChecker < ColumnChecker
+      Report = ReportBuilder.define(
+        DatabaseConsistency::Report,
+        :table_name,
+        :column_name
+      )
+
+      private
+
+      def preconditions
+        column.type == :boolean
+      end
+
+      def check
+        if valid?
+          report_template(:ok)
+        else
+          report_template(:fail, error_slug: :three_state_boolean)
+        end
+      end
+
+      def report_template(status, error_slug: nil)
+        Report.new(
+          status: status,
+          error_slug: error_slug,
+          error_message: nil,
+          table_name: model.table_name,
+          column_name: column.name,
+          **report_attributes
+        )
+      end
+
+      # @return [Boolean]
+      def valid?
+        !column.null
+      end
+    end
+  end
+end
diff --git a/lib/database_consistency/processors/columns_processor.rb b/lib/database_consistency/processors/columns_processor.rb
index e2b483a..e033384 100644
--- a/lib/database_consistency/processors/columns_processor.rb
+++ b/lib/database_consistency/processors/columns_processor.rb
@@ -8,7 +8,8 @@ class ColumnsProcessor < BaseProcessor
         Checkers::NullConstraintChecker,
         Checkers::LengthConstraintChecker,
         Checkers::PrimaryKeyTypeChecker,
-        Checkers::EnumValueChecker
+        Checkers::EnumValueChecker,
+        Checkers::ThreeStateBooleanChecker
       ].freeze
 
       private
diff --git a/lib/database_consistency/writers/autofix_writer.rb b/lib/database_consistency/writers/autofix_writer.rb
index 12a2089..dc54851 100644
--- a/lib/database_consistency/writers/autofix_writer.rb
+++ b/lib/database_consistency/writers/autofix_writer.rb
@@ -16,7 +16,8 @@ class AutofixWriter < BaseWriter
         null_constraint_missing: Autofix::NullConstraintMissing,
         redundant_index: Autofix::RedundantIndex,
         redundant_unique_index: Autofix::RedundantIndex,
-        small_primary_key: Autofix::InconsistentTypes
+        small_primary_key: Autofix::InconsistentTypes,
+        three_state_boolean: Autofix::NullConstraintMissing
       }.freeze
 
       def write
diff --git a/lib/database_consistency/writers/simple/three_state_boolean.rb b/lib/database_consistency/writers/simple/three_state_boolean.rb
new file mode 100644
index 0000000..895caf0
--- /dev/null
+++ b/lib/database_consistency/writers/simple/three_state_boolean.rb
@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module DatabaseConsistency
+  module Writers
+    module Simple
+      class ThreeStateBoolean < Base # :nodoc:
+        private
+
+        def template
+          'boolean column should have NOT NULL constraint'
+        end
+
+        def unique_attributes
+          {
+            table_name: report.table_name,
+            column_name: report.column_name
+          }
+        end
+      end
+    end
+  end
+end
diff --git a/lib/database_consistency/writers/simple_writer.rb b/lib/database_consistency/writers/simple_writer.rb
index 091ef30..96dd783 100644
--- a/lib/database_consistency/writers/simple_writer.rb
+++ b/lib/database_consistency/writers/simple_writer.rb
@@ -28,7 +28,8 @@ class SimpleWriter < BaseWriter
         missing_foreign_key_cascade: Simple::MissingForeignKeyCascade,
         enum_values_inconsistent_with_ar_enum: Simple::EnumValuesInconsistentWithArEnum,
         enum_values_inconsistent_with_inclusion: Simple::EnumValuesInconsistentWithInclusion,
-        redundant_case_insensitive_option: Simple::RedundantCaseInsensitiveOption
+        redundant_case_insensitive_option: Simple::RedundantCaseInsensitiveOption,
+        three_state_boolean: Simple::ThreeStateBoolean
       }.freeze
 
       def write
diff --git a/spec/checkers/three_state_boolean_checker_spec.rb b/spec/checkers/three_state_boolean_checker_spec.rb
new file mode 100644
index 0000000..d47224f
--- /dev/null
+++ b/spec/checkers/three_state_boolean_checker_spec.rb
@@ -0,0 +1,93 @@
+# frozen_string_literal: true
+
+RSpec.describe DatabaseConsistency::Checkers::ThreeStateBooleanChecker, :sqlite, :mysql, :postgresql do
+  subject(:checker) { described_class.new(model, column) }
+
+  let(:klass) { define_class }
+  let(:model) { klass }
+  let(:column) { klass.columns.first }
+
+  context 'when column is nullable and without default' do
+    before do
+      define_database_with_entity do |table|
+        table.boolean :active
+      end
+    end
+
+    specify do
+      expect(checker.report).to have_attributes(
+        checker_name: 'ThreeStateBooleanChecker',
+        table_or_model_name: klass.name,
+        column_or_attribute_name: 'active',
+        status: :fail,
+        error_message: nil,
+        error_slug: :three_state_boolean,
+        table_name: klass.table_name,
+        column_name: 'active'
+      )
+    end
+  end
+
+  context 'when column is not nullable and without default' do
+    before do
+      define_database_with_entity do |table|
+        table.boolean :active, null: false
+      end
+    end
+
+    specify do
+      expect(checker.report).to have_attributes(
+        checker_name: 'ThreeStateBooleanChecker',
+        table_or_model_name: klass.name,
+        column_or_attribute_name: 'active',
+        status: :ok,
+        error_message: nil,
+        error_slug: nil,
+        table_name: klass.table_name,
+        column_name: 'active'
+      )
+    end
+  end
+
+  context 'when column is nullable and with default' do
+    before do
+      define_database_with_entity do |table|
+        table.boolean :active, default: true
+      end
+    end
+
+    specify do
+      expect(checker.report).to have_attributes(
+        checker_name: 'ThreeStateBooleanChecker',
+        table_or_model_name: klass.name,
+        column_or_attribute_name: 'active',
+        status: :fail,
+        error_message: nil,
+        error_slug: :three_state_boolean,
+        table_name: klass.table_name,
+        column_name: 'active'
+      )
+    end
+  end
+
+  context 'when column is not nullable and with default' do
+    before do
+      define_database_with_entity do |table|
+        table.boolean :active, null: false, default: true
+      end
+    end
+
+    specify do
+      expect(checker.report).to have_attributes(
+        checker_name: 'ThreeStateBooleanChecker',
+        table_or_model_name: klass.name,
+        column_or_attribute_name: 'active',
+        status: :ok,
+        error_message: nil,
+        error_slug: nil,
+        table_name: klass.table_name,
+        column_name: 'active'
+      )
+    end
+  end
+end