The ex-Derby County forward took advantage of a defensive mix-up, which saw both centre halves drawn to the ball and with time to pick his spot, side-footed the ball beyond the grasp of keeper Ben Williams.
Indeed, Terry Butcher's men tried hard to force a way back into the game and felt they should have had a penalty when striker James Collins cut in to the penalty area before going down under the challenge of Efe Ambrose, but referee Craig Thomson was unmoved.
Celtic's defending seemed nervy on occasions and the visitors may consider themselves fortunate not to have conceded when a corner rebounded off Virgil van Dijk and rolled into the grateful arms of Forster as Hibs were unable to take advantage.
Towards the end of the half, Samuel Stanton went close to equalising when his skill and strength presented a shooting opportunity on the edge of the area, but he failed to keep his effort down.
Celtic, still without a defeat in the Scottish Premiership this season, would extend their lead at the top of the table to 15 points with a win.