The 37-year-old former England captain, who has joined the wealthy capital club on a five-month deal, replaced Javier Pastore in the 75th minute to the delight of a roaring home crowd.
Beckham immediately produced a trademark long pass to Ezequiel Lavezzi before joining in with a sterling defensive performance.
PSG opened the scoring thanks a bizarre Nicolas Nkoulou own goal in the 12th minute as the ball rolled into the net after Joey Barton and the Cameroon centre back had diverted a Lucas attempt.
Marseille dominated possession but Italy reserve keeper Sirigu made a string of superb saves to preserve the leaders' advantage until Zlatan Ibrahimovic sealed the victory with the second goal from close range in stoppage time.
PSG have 54 points from 26 games, three ahead of Olympique Lyon who earlier beat Lorient 3-1 thanks to goals by Lisandro Lopez, Rachid Ghezzal and Arnold Mvuemba. Marseille stayed third on 46.