The former England captain came off the bench in the 76th minute and played a part in PSG's second goal as they beat Marseille 2-0 in a top-of-the-table clash.
The Parisians are now three points clear of Lyon in Ligue 1, and Beckham was delighted by his impact.
"I'm very happy tonight, mainly because of the victory," Beckham told Sky Sports News. "We have played better but it was very important to win. Today was the perfect day, not only because I'm the 400th player in the history of the club, but also because I'm part of a very special team.
"The welcome from the supporters was unbelievable, and I want to say thank you to them.
"It was a great match, the biggest in France. To have the opportunity at my age to play in such a great championship and with a great team, I am very happy and very proud."
Beckham joined the French outfit on transfer deadline day.