diff --git a/htdocs/compta/facture/class/api_invoices.class.php b/htdocs/compta/facture/class/api_invoices.class.php
index f3aab82422f..2a4e2075b2f 100644
--- a/htdocs/compta/facture/class/api_invoices.class.php
+++ b/htdocs/compta/facture/class/api_invoices.class.php
@@ -562,6 +562,59 @@ class Invoices extends DolibarrApi
       return $updateRes;
     }
 
+    /**
+     * Adds a contact to an invoice
+     *
+     * @param   int 	$id             	Order ID
+     * @param   int 	$fk_socpeople       	Id of thirdparty contact (if source = 'external') or id of user (if souce = 'internal') to link
+     * @param   string 	$type_contact           Type of contact (code). Must a code found into table llx_c_type_contact. For example: BILLING
+     * @param   string  $source             	external=Contact extern (llx_socpeople), internal=Contact intern (llx_user)
+     * @param   int     $notrigger              Disable all triggers 
+     *
+     * @url POST    {id}/contacts
+     *
+     * @return  array
+     *
+     * @throws 200
+     * @throws 304
+     * @throws 401
+     * @throws 404
+     * @throws 500
+     *
+     */
+    function addContact($id, $fk_socpeople, $type_contact, $source, $notrigger=0)
+    {
+        if(! DolibarrApiAccess::$user->rights->facture->creer) {
+                throw new RestException(401);
+        }
+        $result = $this->invoice->fetch($id);
+        if( ! $result ) {
+                throw new RestException(404, 'Invoice not found');
+        }
+
+        if( ! DolibarrApi::_checkAccessToResource('facture',$this->invoice->id)) {
+                throw new RestException(401, 'Access not allowed for login '.DolibarrApiAccess::$user->login);
+        }
+
+        $result = $this->invoice->add_contact($fk_socpeople,$type_contact,$source,$notrigger);
+        if ($result < 0) {
+                throw new RestException(500, 'Error : '.$this->invoice->error);
+        }
+
+        $result = $this->invoice->fetch($id);
+        if( ! $result ) {
+            throw new RestException(404, 'Invoice not found');
+        }
+
+        if( ! DolibarrApi::_checkAccessToResource('facture',$this->invoice->id)) {
+            throw new RestException(401, 'Access not allowed for login '.DolibarrApiAccess::$user->login);
+        }
+
+        return $this->_cleanObjectDatas($this->invoice);
+    }
+
+
+
     /**
      * Sets an invoice as draft
      *