PSG needed only a draw to seal a fifth consecutive Ligue 1 crown and are now nine points ahead of Lens with one game left.
Khvicha Kvaratskhelia scored in the 29th minute and Ibrahim Mbaye added the second goal in added time on Wednesday (Thursday AEST) after Lens missed several chances to score at Stade Bollaert-Delelis.
"Without a doubt this was difficult, Lens did so well this season," PSG coach Luis Enrique said.
"I think they deserved more tonight but (goalkeeper Matvei) Safonov was incredible tonight."
The match was rescheduled from April 11 by the French league to help PSG's ambitions in the Champions League. They take on Premier League leaders Arsenal in the Champions League final on May 30 in Budapest.
Lens also have a final to prepare for — against Nice in the French Cup on May 22 — and Pierre Sage left his two best forwards — wingers Florian Thauvin and Allan Saint-Maximin — on the bench until the hour mark.
The visitors punished a Lens error when Ousmane Dembele intercepted a sloppy pass from defender Malang Sarr and fed Kvaratskhelia, who scored with a clinical finish.
PSG netted again in added time through Mbaye, the 18-year-old forward who thumped a pass from Desire Doue in off the crossbar moments after Lens had fluffed yet another chance.
"We created a lot of chances and stopped them playing. My players deserved at least a draw tonight," Sage said.
"Their goalkeeper had a great game, but we missed a lot of chances. They had two chances and scored."
The last team to stop PSG being crowned champions was Lille in 2021. This season was closer than the last, however, when PSG sealed the title without losing and with six games left.
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