diff --git a/Gemfile b/Gemfile
index bb94df82..0311aedd 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,3 +2,5 @@
 
 source "https://rubygems.org"
 gemspec
+
+gem "sqlite3", "~> 2.9"
diff --git a/authlogic.gemspec b/authlogic.gemspec
index 7eaf0b29..ccec8921 100644
--- a/authlogic.gemspec
+++ b/authlogic.gemspec
@@ -28,6 +28,7 @@ require_relative "lib/authlogic/version"
   s.add_dependency "activerecord", [">= 7.2", "< 8.2"]
   s.add_dependency "activesupport", [">= 7.2", "< 8.2"]
   s.add_dependency "request_store", "~> 1.0"
+  s.add_development_dependency "argon2", "~> 2.0"
   s.add_development_dependency "bcrypt", "~> 3.1"
   s.add_development_dependency "byebug", "~> 11.1.3"
   s.add_development_dependency "coveralls_reborn", "~> 0.29.0"
diff --git a/lib/authlogic/crypto_providers.rb b/lib/authlogic/crypto_providers.rb
index 3e4b4711..753c3594 100644
--- a/lib/authlogic/crypto_providers.rb
+++ b/lib/authlogic/crypto_providers.rb
@@ -27,8 +27,9 @@ module CryptoProviders
     autoload :Sha1,   "authlogic/crypto_providers/sha1"
     autoload :Sha256, "authlogic/crypto_providers/sha256"
     autoload :Sha512, "authlogic/crypto_providers/sha512"
-    autoload :BCrypt, "authlogic/crypto_providers/bcrypt"
-    autoload :SCrypt, "authlogic/crypto_providers/scrypt"
+    autoload :BCrypt,   "authlogic/crypto_providers/bcrypt"
+    autoload :SCrypt,   "authlogic/crypto_providers/scrypt"
+    autoload :Argon2id, "authlogic/crypto_providers/argon2id"
 
     # Guide users to choose a better crypto provider.
     class Guidance
diff --git a/lib/authlogic/crypto_providers/argon2id.rb b/lib/authlogic/crypto_providers/argon2id.rb
new file mode 100644
index 00000000..062025d6
--- /dev/null
+++ b/lib/authlogic/crypto_providers/argon2id.rb
@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+require "argon2"
+
+module Authlogic
+  module CryptoProviders
+    # Argon2id is the recommended variant of the Argon2 password hashing
+    # algorithm, which won the Password Hashing Competition in 2015. It
+    # combines the side-channel resistance of Argon2i with the GPU/ASIC
+    # attack resistance of Argon2d, making it the best choice for
+    # password hashing.
+    #
+    # Argon2id has three configurable cost parameters:
+    #
+    # - t_cost: Number of iterations (time cost). Higher values increase
+    #   computation time. Default: 2
+    # - m_cost: Memory usage in powers of 2 (in kibibytes).
+    #   For example, m_cost of 16 means 2^16 KiB = 64 MiB.
+    #   Default: 16 (64 MiB)
+    # - p_cost: Degree of parallelism (number of threads). Default: 1
+    #
+    # To use Argon2id, install the argon2 gem:
+    #
+    #   gem install argon2
+    #
+    # Tell acts_as_authentic to use it:
+    #
+    #   acts_as_authentic do |c|
+    #     c.crypto_provider = Authlogic::CryptoProviders::Argon2id
+    #   end
+    #
+    # To transition from another provider (lazy migration on login):
+    #
+    #   acts_as_authentic do |c|
+    #     c.crypto_provider = Authlogic::CryptoProviders::Argon2id
+    #     c.transition_from_crypto_providers = [Authlogic::CryptoProviders::SCrypt]
+    #   end
+    #
+    # To update cost parameters (existing passwords are re-hashed on
+    # next login):
+    #
+    #   Authlogic::CryptoProviders::Argon2id.t_cost = 3
+    #   Authlogic::CryptoProviders::Argon2id.m_cost = 17
+    #
+    class Argon2id
+      class << self
+        attr_writer :t_cost, :m_cost, :p_cost
+
+        # Time cost (number of iterations). Default: 2
+        def t_cost
+          @t_cost ||= 2
+        end
+
+        # Memory cost as a power of 2 (in kibibytes). Default: 16 (64 MiB)
+        def m_cost
+          @m_cost ||= 16
+        end
+
+        # Parallelism (number of threads). Default: 1
+        def p_cost
+          @p_cost ||= 1
+        end
+
+        # Creates an Argon2id hash for the password passed.
+        def encrypt(*tokens)
+          hasher = ::Argon2::Password.new(
+            t_cost: t_cost,
+            m_cost: m_cost,
+            p_cost: p_cost
+          )
+          hasher.create(join_tokens(tokens))
+        end
+
+        # Does the hash match the tokens? Uses the same tokens that were
+        # used to encrypt.
+        def matches?(hash, *tokens)
+          return false if hash.blank?
+          ::Argon2::Password.verify_password(join_tokens(tokens), hash)
+        rescue ::Argon2::ArgonHashFail
+          false
+        end
+
+        # Checks whether the existing hash uses the same cost parameters
+        # as the current configuration. If not, Authlogic will re-hash
+        # the password on next successful login.
+        def cost_matches?(hash)
+          return false if hash.blank?
+          params = extract_params(hash)
+          return false if params.nil?
+          params[:t] == t_cost &&
+            params[:m] == (1 << m_cost) &&
+            params[:p] == p_cost
+        end
+
+        private
+
+        def join_tokens(tokens)
+          tokens.flatten.join
+        end
+
+        # Parses cost parameters from an Argon2id hash string.
+        # Format: $argon2id$v=19$m=65536,t=2,p=1$salt$hash
+        def extract_params(hash)
+          match = hash.to_s.match(/\$argon2id?\$v=\d+\$m=(\d+),t=(\d+),p=(\d+)\$/)
+          return nil unless match
+          { m: match[1].to_i, t: match[2].to_i, p: match[3].to_i }
+        end
+      end
+    end
+  end
+end
diff --git a/test/acts_as_authentic_test/password_test.rb b/test/acts_as_authentic_test/password_test.rb
index e13c76dd..bb905d0e 100644
--- a/test/acts_as_authentic_test/password_test.rb
+++ b/test/acts_as_authentic_test/password_test.rb
@@ -92,6 +92,49 @@ def test_transitioning_password
       )
     end
 
+    def test_transitioning_password_to_argon2id
+      ben = users(:ben)
+      transition_password_to(Authlogic::CryptoProviders::Argon2id, ben)
+    end
+
+    def test_transitioning_password_from_argon2id
+      ben = users(:ben)
+      transition_password_to(Authlogic::CryptoProviders::Argon2id, ben)
+      transition_password_to(
+        Authlogic::CryptoProviders::SCrypt,
+        ben,
+        Authlogic::CryptoProviders::Argon2id
+      )
+    end
+
+    def test_argon2id_cost_migration
+      ben = users(:aaron)
+      original_t_cost = Authlogic::CryptoProviders::Argon2id.t_cost
+
+      # Set up user with Argon2id
+      User.acts_as_authentic do |c|
+        c.crypto_provider = Authlogic::CryptoProviders::Argon2id
+        c.transition_from_crypto_providers = []
+      end
+      ben.password = "aaronrocks"
+      ben.password_confirmation = "aaronrocks"
+      ben.save(validate: false)
+
+      old_hash = ben.crypted_password
+      assert Authlogic::CryptoProviders::Argon2id.cost_matches?(old_hash)
+
+      # Increase cost
+      Authlogic::CryptoProviders::Argon2id.t_cost = original_t_cost + 1
+      refute Authlogic::CryptoProviders::Argon2id.cost_matches?(old_hash)
+
+      # On next valid_password?, it should re-hash
+      assert ben.valid_password?("aaronrocks")
+      assert_not_equal old_hash, ben.crypted_password
+      assert Authlogic::CryptoProviders::Argon2id.cost_matches?(ben.crypted_password)
+    ensure
+      Authlogic::CryptoProviders::Argon2id.t_cost = original_t_cost
+    end
+
     def test_v2_crypto_provider_transition
       ben = users(:ben)
 
diff --git a/test/crypto_provider_test/argon2id_test.rb b/test/crypto_provider_test/argon2id_test.rb
new file mode 100644
index 00000000..f2d34900
--- /dev/null
+++ b/test/crypto_provider_test/argon2id_test.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module CryptoProviderTest
+  class Argon2idTest < ActiveSupport::TestCase
+    def test_encrypt
+      assert Authlogic::CryptoProviders::Argon2id.encrypt("mypass")
+    end
+
+    def test_encrypt_produces_argon2id_hash
+      hash = Authlogic::CryptoProviders::Argon2id.encrypt("mypass")
+      assert hash.start_with?("$argon2id$")
+    end
+
+    def test_matches
+      hash = Authlogic::CryptoProviders::Argon2id.encrypt("mypass")
+      assert Authlogic::CryptoProviders::Argon2id.matches?(hash, "mypass")
+    end
+
+    def test_does_not_match_wrong_password
+      hash = Authlogic::CryptoProviders::Argon2id.encrypt("mypass")
+      refute Authlogic::CryptoProviders::Argon2id.matches?(hash, "wrongpass")
+    end
+
+    def test_does_not_match_blank_hash
+      refute Authlogic::CryptoProviders::Argon2id.matches?("", "mypass")
+      refute Authlogic::CryptoProviders::Argon2id.matches?(nil, "mypass")
+    end
+
+    def test_matches_with_multiple_tokens
+      hash = Authlogic::CryptoProviders::Argon2id.encrypt("mypass", "salt123")
+      assert Authlogic::CryptoProviders::Argon2id.matches?(hash, "mypass", "salt123")
+      refute Authlogic::CryptoProviders::Argon2id.matches?(hash, "mypass", "othersalt")
+    end
+
+    def test_cost_matches_with_current_params
+      hash = Authlogic::CryptoProviders::Argon2id.encrypt("mypass")
+      assert Authlogic::CryptoProviders::Argon2id.cost_matches?(hash)
+    end
+
+    def test_cost_does_not_match_after_t_cost_change
+      hash = Authlogic::CryptoProviders::Argon2id.encrypt("mypass")
+      original_t_cost = Authlogic::CryptoProviders::Argon2id.t_cost
+      begin
+        Authlogic::CryptoProviders::Argon2id.t_cost = original_t_cost + 1
+        refute Authlogic::CryptoProviders::Argon2id.cost_matches?(hash)
+      ensure
+        Authlogic::CryptoProviders::Argon2id.t_cost = original_t_cost
+      end
+    end
+
+    def test_cost_does_not_match_blank_hash
+      refute Authlogic::CryptoProviders::Argon2id.cost_matches?("")
+      refute Authlogic::CryptoProviders::Argon2id.cost_matches?(nil)
+    end
+
+    def test_does_not_match_invalid_hash
+      refute Authlogic::CryptoProviders::Argon2id.matches?("not_a_real_hash", "mypass")
+    end
+  end
+end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index f2e3de8c..01e0303b 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -163,6 +163,11 @@
 Authlogic::CryptoProviders::SCrypt.max_time = 0.001 # 1ms
 Authlogic::CryptoProviders::SCrypt.max_mem = 1024 * 1024 # 1MB, the minimum SCrypt allows
 
+# Configure Argon2id to be as fast as possible for tests.
+Authlogic::CryptoProviders::Argon2id.t_cost = 1
+Authlogic::CryptoProviders::Argon2id.m_cost = 3 # 2^3 = 8 KiB, minimum Argon2 allows
+Authlogic::CryptoProviders::Argon2id.p_cost = 1
+
 require "libs/project"
 require "libs/affiliate"
 require "libs/employee"