diff --git a/app/Ninja/Repositories/InvoiceRepository.php b/app/Ninja/Repositories/InvoiceRepository.php
index 72e063d89f26..229963b097b4 100644
--- a/app/Ninja/Repositories/InvoiceRepository.php
+++ b/app/Ninja/Repositories/InvoiceRepository.php
@@ -402,6 +402,8 @@ class InvoiceRepository extends BaseRepository
 
         if ($invoice->is_deleted) {
             return $invoice;
+        } elseif ($invoice->isSent() && config('ninja.lock_sent_invoices')) {
+            return $invoice;
         }
 
         if (isset($data['has_tasks']) && filter_var($data['has_tasks'], FILTER_VALIDATE_BOOLEAN)) {
diff --git a/config/ninja.php b/config/ninja.php
index 402d9bfc9d3e..102820edeec9 100644
--- a/config/ninja.php
+++ b/config/ninja.php
@@ -2,6 +2,9 @@
 
 return [
 
+    // invoice locking feature
+    'lock_sent_invoices' => env('LOCK_SENT_INVOICES'),
+
     // Marketing links
     'time_tracker_web_url' => env('TIME_TRACKER_WEB_URL', 'https://www.invoiceninja.com/time-tracker'),
 
diff --git a/docs/configure.rst b/docs/configure.rst
index 8c4f8dff21a3..a14e9cfb382b 100644
--- a/docs/configure.rst
+++ b/docs/configure.rst
@@ -123,6 +123,11 @@ You'll also need to set the following values in the .env file.
    MSBOT_LUIS_APP_ID=...
    MSBOT_LUIS_SUBSCRIPTION_KEY=...
 
+Lock Invoices
+"""""""""""""
+
+Adding ``LOCK_SENT_INVOICES=true`` to the .env file will prevent changing an invoice once it has been sent.
+
 Using a Proxy
 """""""""""""
 
diff --git a/resources/views/invoices/edit.blade.php b/resources/views/invoices/edit.blade.php
index ffff300bba08..b17375bfab2e 100644
--- a/resources/views/invoices/edit.blade.php
+++ b/resources/views/invoices/edit.blade.php
@@ -522,8 +522,10 @@
         @if (Auth::user()->canCreateOrEdit(ENTITY_INVOICE, $invoice))
             @if ($invoice->isClientTrashed())
                 <!-- do nothing -->
+			@elseif ($invoice->isSent() && config('ninja.lock_sent_invoices'))
+				<!-- do nothing -->
             @else
-                @if (!$invoice->is_deleted)
+				@if (!$invoice->is_deleted)
 					@if ($invoice->isSent())
 						{!! Button::success(trans("texts.save_{$entityType}"))->withAttributes(array('id' => 'saveButton', 'onclick' => 'onSaveClick()'))->appendIcon(Icon::create('floppy-disk')) !!}
 					@else
@@ -1375,6 +1377,10 @@
             return false;
         }
 
+		@if ($invoice->isSent() && config('ninja.lock_sent_invoices'))
+			return false;
+		@endif
+
         @if ($invoice->is_deleted || $invoice->isClientTrashed())
             if ($('#bulk_action').val() != 'restore') {
                 return false;