diff --git a/lib/devise.rb b/lib/devise.rb
index 8e0c85e77d..2e3f7ce705 100644
--- a/lib/devise.rb
+++ b/lib/devise.rb
@@ -273,14 +273,8 @@ module Test
   # PRIVATE CONFIGURATION
 
   # Store scopes mappings.
+  mattr_accessor :mappings
   @@mappings = {}
-  def self.mappings
-    # Starting from Rails 8.0, routes are lazy-loaded by default in test and development environments.
-    # However, Devise's mappings are built during the routes loading phase.
-    # To ensure it works correctly, we need to load the routes first before accessing @@mappings.
-    Rails.application.try(:reload_routes_unless_loaded)
-    @@mappings
-  end
 
   # OmniAuth configurations.
   mattr_reader :omniauth_configs
@@ -360,11 +354,29 @@ def self.mailer=(class_name)
   end
   self.mailer = "Devise::Mailer"
 
-  # Small method that adds a mapping to Devise.
+  # Registers a mapping with Devise. This initializes a mapping that only
+  # contains information about the model; routing information must be added
+  # later (typically while routes are loaded) via
+  # `Devise::Mapping#add_routes_options!`.
+  #
+  # This is now idempotent, so that `devise_for` can still create a mapping if
+  # it was not initialized before route loading, or use the existing one if it
+  # was previously initialized.
   def self.add_mapping(resource, options)
+    _scoped_path, name = Devise::Mapping.mapping_name(resource, as: options[:as], singular: options[:singular])
+
+    if (mapping = @@mappings[name])
+      requested = (options[:class_name] || resource.to_s.classify).to_s
+      if options.key?(:class_name) && requested != mapping.class_name
+        raise ArgumentError, "conflicting class_name for the #{name.inspect} scope: " \
+          "already mapped to #{mapping.class_name.inspect} but got #{requested.inspect}"
+      end
+      return mapping
+    end
+
     mapping = Devise::Mapping.new(resource, options)
-    @@mappings[mapping.name] = mapping
-    @@default_scope ||= mapping.name
+    @@mappings[name] = mapping
+    @@default_scope ||= name
     @@helpers.each { |h| h.define_helpers(mapping) }
     mapping
   end
diff --git a/lib/devise/mapping.rb b/lib/devise/mapping.rb
index 8b1f94ced2..1c6e1616c9 100644
--- a/lib/devise/mapping.rb
+++ b/lib/devise/mapping.rb
@@ -47,18 +47,26 @@ def self.find_scope!(obj)
     end
 
     def self.find_by_path!(path, path_type = :fullpath)
-      Devise.mappings.each_value { |m| return m if path.include?(m.send(path_type)) }
+      Devise.mappings.each_value { |m| return m if m.finalized? && path.include?(m.send(path_type)) }
       raise "Could not find a valid mapping for path #{path.inspect}"
     end
 
+    def self.mapping_name(resource, as: nil, singular: nil)
+      scoped_path = as ? "#{as}/#{resource}" : resource.to_s
+      singular = (singular || scoped_path.tr('/', '_').singularize).to_sym
+      [scoped_path, singular]
+    end
+
     def initialize(name, options) #:nodoc:
-      @scoped_path = options[:as] ? "#{options[:as]}/#{name}" : name.to_s
-      @singular = (options[:singular] || @scoped_path.tr('/', '_').singularize).to_sym
+      @resource = name
+      @scoped_path, @singular = self.class.mapping_name(name, as: options[:as], singular: options[:singular])
 
       @class_name = (options[:class_name] || name.to_s.classify).to_s
       @klass = Devise.ref(@class_name)
+    end
 
-      @path = (options[:path] || name).to_s
+    def add_routes_options!(options)
+      @path = (options[:path] || @resource).to_s
       @path_prefix = options[:path_prefix]
 
       @sign_out_via = options[:sign_out_via] || Devise.sign_out_via
@@ -71,6 +79,11 @@ def initialize(name, options) #:nodoc:
       default_path_names(options)
       default_used_route(options)
       default_used_helpers(options)
+      @finalized = true
+    end
+
+    def finalized?
+      !!@finalized
     end
 
     # Return modules for the mapping.
diff --git a/lib/devise/rails.rb b/lib/devise/rails.rb
index b5738853fe..958cd1b259 100644
--- a/lib/devise/rails.rb
+++ b/lib/devise/rails.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'devise/rails/routes'
+require 'devise/rails/devise_model'
 require 'devise/rails/warden_compat'
 
 module Devise
@@ -17,6 +18,10 @@ class Engine < ::Rails::Engine
       app.reload_routes! if Devise.reload_routes
     end
 
+    config.after_initialize do
+      Devise.configure_warden!
+    end
+
     initializer "devise.deprecator" do |app|
       app.deprecators[:devise] = Devise.deprecator if app.respond_to?(:deprecators)
     end
diff --git a/lib/devise/rails/devise_model.rb b/lib/devise/rails/devise_model.rb
new file mode 100644
index 0000000000..c33c702bb5
--- /dev/null
+++ b/lib/devise/rails/devise_model.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Devise
+  ALLOWED_DEVISE_MODEL_OPTIONS = [:class_name, :singular, :as].freeze
+
+  # Registers the model phase of a Devise mapping during Rails initialization,
+  # before routes are loaded. This makes Devise.mappings[scope] available early
+  # (with its model, scope and strategies) even when routes are lazy-loaded,
+  # which Rails does by default in development and test starting from Rails 8.
+  #
+  #   Devise.devise_model :user
+  #   Devise.devise_model :admin, class_name: "Account", singular: :manager
+  def self.devise_model(resource, options = {})
+    options = options.symbolize_keys
+    invalid_options = options.keys - ALLOWED_DEVISE_MODEL_OPTIONS
+
+    unless invalid_options.empty?
+      raise ArgumentError, "Devise.devise_model only accepts model options " \
+        "(#{ALLOWED_DEVISE_MODEL_OPTIONS.join(', ')}); got: #{invalid_options.join(', ')}. " \
+        "Routing options must be given to devise_for."
+    end
+
+    add_mapping(resource, options)
+  end
+end
diff --git a/lib/devise/rails/routes.rb b/lib/devise/rails/routes.rb
index f43e62fea7..5b249d891f 100644
--- a/lib/devise/rails/routes.rb
+++ b/lib/devise/rails/routes.rb
@@ -16,7 +16,6 @@ def finalize!
             " to :main_app as well in case you want to keep the current behavior."
         end
 
-        Devise.configure_warden!
         Devise.regenerate_helpers!
         true
       end
@@ -240,6 +239,7 @@ def devise_for(*resources)
 
       resources.each do |resource|
         mapping = Devise.add_mapping(resource, options)
+        mapping.add_routes_options!(options)
 
         begin
           raise_no_devise_method_error!(mapping.class_name) unless mapping.to.respond_to?(:devise)
diff --git a/test/mapping_test.rb b/test/mapping_test.rb
index 9d60287cd4..839e531efc 100644
--- a/test/mapping_test.rb
+++ b/test/mapping_test.rb
@@ -133,4 +133,60 @@ def user.devise_scope; :special_scope; end
       Devise::Mapping.find_by_path!('/accounts/facebook/callback', :path)
     end
   end
+
+  test 'mapping_name computes the scoped path and scope from the resource' do
+    assert_equal ["users", :user], Devise::Mapping.mapping_name(:users)
+    assert_equal ["user", :user],  Devise::Mapping.mapping_name(:user)
+  end
+
+  test 'mapping_name honors the :singular option' do
+    assert_equal ["accounts", :manager], Devise::Mapping.mapping_name(:accounts, singular: :manager)
+  end
+
+  test 'mapping_name honors the :as option' do
+    assert_equal ["publisher/account", :publisher_account], Devise::Mapping.mapping_name(:account, as: "publisher")
+  end
+
+  test 'a mapping built from devise_for is finalized' do
+    assert Devise.mappings[:user].finalized?
+  end
+
+  test 'a partial mapping exposes the model phase but is not finalized' do
+    mapping = Devise::Mapping.new(:partial_admin, class_name: "Admin", singular: :partial_admin)
+
+    assert_not mapping.finalized?
+    assert_equal Admin,    mapping.to
+    assert_equal :partial_admin, mapping.name
+    assert_equal Admin.devise_modules, mapping.modules
+    assert_equal [:database_authenticatable], mapping.strategies
+  end
+
+  test 'add_routes_options! finalizes a partial mapping with the routing options' do
+    mapping = Devise::Mapping.new(:partial_admin, class_name: "Admin")
+    mapping.add_routes_options!(path: "admins_area", sign_out_via: :get)
+
+    assert mapping.finalized?
+    assert_equal "admins_area", mapping.path
+    assert_equal :get,          mapping.sign_out_via
+    assert_not_nil mapping.used_routes
+  end
+
+  test 'add_routes_options! defaults the path from the resource, not the singular' do
+    mapping = Devise::Mapping.new(:accounts, class_name: "Admin", singular: :manager)
+    mapping.add_routes_options!({})
+
+    assert_equal "accounts", mapping.path
+    assert_equal :manager,   mapping.name
+  end
+
+  test 'find_by_path! ignores mappings that are not finalized' do
+    partial = Devise::Mapping.new(:partial_admin, class_name: "Admin")
+    Devise.mappings[:partial_admin] = partial
+
+    assert_raise RuntimeError do
+      Devise::Mapping.find_by_path!("/partial_admin/sign_in")
+    end
+  ensure
+    Devise.mappings.delete(:partial_admin)
+  end
 end
diff --git a/test/rails_app/config/initializers/devise.rb b/test/rails_app/config/initializers/devise.rb
index 7414bc1f94..2075996e5c 100644
--- a/test/rails_app/config/initializers/devise.rb
+++ b/test/rails_app/config/initializers/devise.rb
@@ -15,6 +15,12 @@
   config.secret_key = "d9eb5171c59a4c817f68b0de27b8c1e340c2341b52cdbc60d3083d4e8958532" \
                       "18dcc5f589cafde048faec956b61f864b9b5513ff9ce29bf9e5d58b0f234f8e3b"
 
+  # An example of configuring a devise-enabled model via early initialization,
+  # before route loading. This same resource name still needs to be configured
+  # again in the routes, via `devise_for`, in order to register its routes and
+  # finalize the mapping configuration.
+  config.devise_model :users
+
   # ==> Mailer Configuration
   # Configure the e-mail address which will be shown in Devise::Mailer,
   # note that it will be overwritten if you use your own mailer class with default "from" parameter.
diff --git a/test/routes_test.rb b/test/routes_test.rb
index 20ba311727..0db2b22499 100644
--- a/test/routes_test.rb
+++ b/test/routes_test.rb
@@ -274,3 +274,54 @@ class ScopedRoutingTest < ActionController::TestCase
     assert_equal '/publisher/accounts/get_in', @routes.url_helpers.new_publisher_account_session_path
   end
 end
+
+class DeviseModelTest < ActiveSupport::TestCase
+  test 'registers a partial mapping that is not finalized' do
+    mapping = Devise.devise_model :registered_admin, class_name: "Admin"
+
+    assert_equal mapping, Devise.mappings[:registered_admin]
+    assert_not mapping.finalized?
+    assert_equal Admin, mapping.to
+  ensure
+    Devise.mappings.delete(:registered_admin)
+  end
+
+  test 'derives the class_name from the resource name when not given' do
+    mapping = Devise.devise_model :registered_admin
+
+    assert_equal "RegisteredAdmin", mapping.class_name
+  ensure
+    Devise.mappings.delete(:registered_admin)
+  end
+
+  test 'is idempotent for an already registered scope' do
+    first  = Devise.devise_model :registered_admin, class_name: "Admin"
+    second = Devise.devise_model :registered_admin, class_name: "Admin"
+
+    assert_same first, second
+  ensure
+    Devise.mappings.delete(:registered_admin)
+  end
+
+  test 'rejects routing options' do
+    e = assert_raise ArgumentError do
+      Devise.devise_model :registered_admin, path: "admins_area"
+    end
+    assert_match "only accepts model options", e.message
+    assert_nil Devise.mappings[:registered_admin]
+  end
+
+  test 'devise_for raises on a class_name conflicting with the partial mapping' do
+    Devise.devise_model :registered_admin, class_name: "Admin"
+
+    e = assert_raise ArgumentError do
+      routes = ActionDispatch::Routing::RouteSet.new
+      routes.draw do
+        devise_for :registered_admin, class_name: "User"
+      end
+    end
+    assert_match "conflicting class_name", e.message
+  ensure
+    Devise.mappings.delete(:registered_admin)
+  end
+end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index e6df812037..ccbcc561f9 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -10,6 +10,11 @@
 require "rails/test_help"
 require "orm/#{DEVISE_ORM}"
 
+# Routes are lazy-loaded by default in development and test from Rails 8, so the
+# Devise mappings declared by devise_for are not built until something loads the
+# routes. Load them now so unit tests can rely on Devise.mappings being present.
+Rails.application.reload_routes_unless_loaded
+
 I18n.load_path.concat Dir["#{File.dirname(__FILE__)}/support/locale/*.yml"]
 
 require 'mocha/minitest'