Early in the first half, Azpilicueta hit the post with a deflected shot before Lukaku put the Blues ahead, but Brighton came alive after that.
Mendy made some excellent saves to deny Lallana, Mac Allister, and Bissouma, with Moder’s chance shortly after the interval clipping the crossbar.
They persisted, and Welbeck’s stoppage-time header earned them a point.
Chelsea has dropped more points, however they have moved ahead of Liverpool, who they will face next time out, into second place.
With Manchester City grabbing 1-0 win in their game, they will be seven points behind the leaders as it stands.
With yet another draw, Brighton climb to tenth place, and they head to Everton on Sunday.