diff --git a/app/controllers/accounting_posts_controller.rb b/app/controllers/accounting_posts_controller.rb
index cf11ec7c3..88a3015c8 100644
--- a/app/controllers/accounting_posts_controller.rb
+++ b/app/controllers/accounting_posts_controller.rb
@@ -14,7 +14,7 @@ class AccountingPostsController < CrudController
   self.permitted_attrs = [:closed, :offered_hours, :offered_rate, :offered_total,
                           :remaining_hours, :portfolio_item_id, :service_id, :billable,
                           :description_required, :ticket_required, :from_to_times_required,
-                          :meal_compensation,
+                          :meal_compensation, :billing_reminder_active,
                           { work_item_attributes: %i[name shortname description] }]
 
   helper_method :order
diff --git a/app/controllers/worktimes_controller.rb b/app/controllers/worktimes_controller.rb
index c50710734..67c41d41a 100644
--- a/app/controllers/worktimes_controller.rb
+++ b/app/controllers/worktimes_controller.rb
@@ -13,6 +13,8 @@ class WorktimesController < CrudController
   before_save :check_has_accounting_post, :check_worktimes_committed
   after_save :check_overlapping, :check_employment
 
+  before_action :set_unbilled_worktimes_popup
+
   before_render_index :set_statistics
   before_render_new :create_default_worktime
   before_render_form :set_existing
@@ -177,6 +179,20 @@ def set_statistics
     @remaining_vacations = @user.statistics.current_remaining_vacations
   end
 
+  def set_unbilled_worktimes_popup
+    return if Time.zone.today.day < 10 # only show from 10. day of the month onwards to give time to publish / control times
+
+    accounting_posts = @user.managed_orders
+                            .collect(&:accounting_posts)
+                            .flatten
+                            .filter { |ap| ap.billing_reminder_active == true && ap.unbilled_billable_times_exist_in_past_month? }
+
+    return if accounting_posts.blank?
+
+    reminder_text = 'Vorsicht, in einem oder meheren deiner Aufträge gibt es noch unverrechnete Leistungen vom Vormonat! Überprüfe bitte das Verrechnungs-Controlling.'
+    flash[:warning] = reminder_text
+  end
+
   # returns the employee's id from the params or the logged in user
   def employee_id
     if record_other?
diff --git a/app/jobs/not_billed_times_reminder_job.rb b/app/jobs/not_billed_times_reminder_job.rb
new file mode 100644
index 000000000..1bfa3ba67
--- /dev/null
+++ b/app/jobs/not_billed_times_reminder_job.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+#  Copyright (c) 2006-2022, Puzzle ITC GmbH. This file is part of
+#  PuzzleTime and licensed under the Affero General Public License version 3
+#  or later. See the COPYING file at the top-level directory or at
+#  https://github.com/puzzle/puzzletime.
+
+class NotBilledTimesReminderJob < CronJob
+  self.cron_expression = '0 5 10 * *'
+
+  def perform
+    Employee.active_employed_last_month.each do |employee|
+      accounting_posts = employee.managed_orders
+                                 .collect(&:accounting_posts)
+                                 .flatten
+                                 .filter { |ap| ap.billing_reminder_active == true }
+      EmployeeMailer.not_billed_times_reminder_mail(employee).deliver_now if accounting_posts.any?(&:unbilled_billable_times_exist_in_past_month?)
+    end
+  end
+end
diff --git a/app/mailers/employee_mailer.rb b/app/mailers/employee_mailer.rb
index a58d0832e..f98bfd0f6 100644
--- a/app/mailers/employee_mailer.rb
+++ b/app/mailers/employee_mailer.rb
@@ -17,4 +17,13 @@ def worktime_commit_reminder_mail(employee)
       subject: 'PuzzleTime Zeiten freigeben'
     )
   end
+
+  def not_billed_times_reminder_mail(employee)
+    @employee = employee
+
+    mail(
+      to: "#{employee.firstname} #{employee.lastname} <#{employee.email}>",
+      subject: 'Erinnerung: Noch nicht verrechnete PuzzleTime Zeiten'
+    )
+  end
 end
diff --git a/app/models/accounting_post.rb b/app/models/accounting_post.rb
index aff2f9037..80a485922 100644
--- a/app/models/accounting_post.rb
+++ b/app/models/accounting_post.rb
@@ -102,6 +102,14 @@ def propagate_closed!
     work_item.propagate_closed!(order.status.closed? || closed?)
   end
 
+  def unbilled_billable_times_exist_in_past_month?
+    work_item.worktimes
+             .in_period(Period.parse('-1m'))
+             .where(billable: true)
+             .where(invoice_id: nil)
+             .present?
+  end
+
   private
 
   def derive_offered_fields
diff --git a/app/views/accounting_posts/_form.html.haml b/app/views/accounting_posts/_form.html.haml
index f0db1aa5f..06edebc01 100644
--- a/app/views/accounting_posts/_form.html.haml
+++ b/app/views/accounting_posts/_form.html.haml
@@ -44,3 +44,4 @@
       .col-md-2
       .help-block.col-md-5
         Die Einstellungen zur Beschreibung, Ticket oder Von-Bis-Zeiten können nicht geändert werden, da bereits Leistungen ohne diese Angaben erfasst wurden.
+  = f.labeled_input_field :billing_reminder_active, html_options = {caption: 'Erinnerung bei unverrechneten Leistungen senden', title: "Am 10. Tag des Monats wird eine Mail an den/die Auftragsverantwortliche:n gesendet, wenn im Vormonat verrechenbare Leistungen auf diese Position gebucht wurden, die keiner Rechnung zugewiesen sind", data: {toggle: :tooltip}}
diff --git a/app/views/employee_mailer/not_billed_times_reminder_mail.html.haml b/app/views/employee_mailer/not_billed_times_reminder_mail.html.haml
new file mode 100644
index 000000000..aec2bd0b0
--- /dev/null
+++ b/app/views/employee_mailer/not_billed_times_reminder_mail.html.haml
@@ -0,0 +1,16 @@
+%h1.h3{:style => "box-sizing: border-box; margin: 0.67em 0; font-family: inherit; font-weight: 400; line-height: 1.1; color: inherit; margin-top: 20px; font-size: 24px; margin-bottom: 20px;"}
+  Hallo
+  = @employee.firstname
+%div{:style => "box-sizing: border-box;"}
+  .lead
+    Beim einem der Aufträge, bei welchen du Auftragsverantwortliche:r bist, wurden im vergangenen Monat verrechenbare Leistungen gebucht, welche noch keiner Rechnung zugeteilt wurden.
+    %br
+    Bitte überprüfe die Leistungen im
+    = link_to 'Verrechnungs-Controlling', root_url
+  %br
+  .lead
+    Liebe Grüsse
+    %br
+    Dein PuzzleTime
+  %br
+    Möchtest du zu einer Buchungsposition künftig keine Erinnerungsmail mehr erhalten, deaktiviere in den Einstellungen der Position die Checkbox "Erinnerung bei unverrechneten Leistungen senden".
\ No newline at end of file
diff --git a/app/views/employee_mailer/not_billed_times_reminder_mail.text.erb b/app/views/employee_mailer/not_billed_times_reminder_mail.text.erb
new file mode 100644
index 000000000..0ef931735
--- /dev/null
+++ b/app/views/employee_mailer/not_billed_times_reminder_mail.text.erb
@@ -0,0 +1,10 @@
+Hallo <%= @employee.firstname %>
+
+Beim einem der Aufträge, bei welchen du Auftragsverantwortliche:r bist, wurden im vergangenen Monat verrechenbare Leistungen gebucht, welche noch keiner Rechnung zugeteilt wurden.
+
+Bitte überprüfe die Leistungen im 'Verrechnungs-Controlling' im PuzzleTime.
+
+Möchtest du zu einer Buchungsposition künftig keine Erinnerungsmail mehr erhalten, deaktiviere in den Einstellungen der Position die Checkbox "Erinnerung bei unverrechneten Leistungen senden".
+
+Liebe Grüsse
+Dein PuzzleTime
\ No newline at end of file
diff --git a/db/migrate/20250501075712_add_billing_reminder_active_to_accounting_posts.rb b/db/migrate/20250501075712_add_billing_reminder_active_to_accounting_posts.rb
new file mode 100644
index 000000000..d164c8789
--- /dev/null
+++ b/db/migrate/20250501075712_add_billing_reminder_active_to_accounting_posts.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddBillingReminderActiveToAccountingPosts < ActiveRecord::Migration[7.1]
+  def change
+    add_column :accounting_posts, :billing_reminder_active, :boolean, default: true, null: false
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 1bd87b11b..9ed7f64f0 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2025_02_19_080518) do
+ActiveRecord::Schema[7.1].define(version: 2025_05_01_075712) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
 
@@ -35,6 +35,7 @@
     t.boolean "closed", default: false, null: false
     t.integer "service_id"
     t.boolean "meal_compensation", default: false, null: false
+    t.boolean "billing_reminder_active", default: true, null: false
     t.index ["portfolio_item_id"], name: "index_accounting_posts_on_portfolio_item_id"
     t.index ["service_id"], name: "index_accounting_posts_on_service_id"
     t.index ["work_item_id"], name: "index_accounting_posts_on_work_item_id"
diff --git a/test/mailers/employee_mailer_test.rb b/test/mailers/employee_mailer_test.rb
new file mode 100644
index 000000000..e3a48932a
--- /dev/null
+++ b/test/mailers/employee_mailer_test.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class EmployeeMailerTest < ActionMailer::TestCase
+  attr_reader :order, :accounting_post1, :accounting_post2
+
+  setup do
+    @order = Fabricate(:order,
+                       responsible: employees(:next_year_pablo))
+    @accounting_post1 = Fabricate(:accounting_post,
+                                  work_item: Fabricate(:work_item, parent_id: order.work_item_id),
+                                  offered_hours: 100,
+                                  offered_rate: 100,
+                                  billing_reminder_active: true)
+    @accounting_post2 = Fabricate(:accounting_post,
+                                  work_item: Fabricate(:work_item, parent_id: order.work_item_id),
+                                  offered_hours: 100,
+                                  offered_rate: 0,
+                                  billing_reminder_active: false)
+  end
+
+  test 'sends a reminder for an order responsible with active employment' do
+    order_responsible = employees(:next_year_pablo)
+    Fabricate(:ordertime, work_item: accounting_post1.work_item, employee: employees(:long_time_john), hours: 5, billable: true, work_date: Period.parse('-1m').end_date)
+
+    assert_emails 1 do
+      NotBilledTimesReminderJob.new.perform
+    end
+    mail = ActionMailer::Base.deliveries.last
+
+    assert_equal [order_responsible.email], mail.to
+  end
+
+  test 'setting `billing_reminder_active: false` deactivates mails for an accounting_post' do
+    Fabricate(:ordertime, work_item: accounting_post2.work_item, employee: employees(:long_time_john), hours: 7, billable: true, work_date: Period.parse('-1m').end_date)
+
+    assert_emails 0 do
+      NotBilledTimesReminderJob.new.perform
+    end
+  end
+end
diff --git a/test/mailers/previews/employee_mailer_preview.rb b/test/mailers/previews/employee_mailer_preview.rb
index afbe648eb..7df6c2c4d 100644
--- a/test/mailers/previews/employee_mailer_preview.rb
+++ b/test/mailers/previews/employee_mailer_preview.rb
@@ -18,4 +18,9 @@ def worktime_commit_reminder_mail
     employee = Employee.new(email: 'user@example.com', firstname: 'Peter', lastname: 'Puzzler')
     EmployeeMailer.worktime_commit_reminder_mail(employee)
   end
+
+  def not_billed_times_reminder_mail
+    employee = Employee.new(email: 'user@example.com', firstname: 'Peter', lastname: 'Puzzler')
+    EmployeeMailer.not_billed_times_reminder_mail(employee)
+  end
 end