Koscielny was red-carded after holding back Nile Ranger when he was clean through on goal.
FA: "At a regulatory commission hearing, a claim for wrongful dismissal from Arsenal's Laurent Koscielny was dismissed.
"As a result, Koscielny will serve a two-match suspension with immediate effect as this was his second dismissal of the season.
"Koscielny was dismissed for denying an obvious goalscoring opportunity against Newcastle on 7 November."