Bradley Barcola opened the scoring with a sharp finish at the near post, while Khvicha Kvaratskhelia doubled the lead before halftime, tapping in from close range. Joao Neves came close to adding another when his shot struck the crossbar, but Vitinha made no mistake, curling a precise effort into the bottom corner from 18 yards just before the hour mark.
PSG then shifted into high gear. Goncalo Ramos set up substitute Desire Doue for the fourth, Nuno Mendes latched onto Achraf Hakimi's cross for the fifth, and Ramos added a sixth with a delicate finish. Teenager Senny Mayulu capped off the scoring with four minutes remaining, completing a night of misery for Brest. The visitors had few chances, with Mathias Pereira Lage's effort cleared off the line by Marquinhos and Pierre Lees-Melou hitting the post.
PSG now awaits their next opponent, with the draw set for Friday, February 21, in Nyon, Switzerland. They will face either Liverpool or Barcelona in the next round.
Meanwhile, Borussia Dortmund also booked their place in the last 16, comfortably managing their second-leg fixture after a 3-0 victory over Sporting CP in the first leg. Goals from Serhou Guirassy, Pascal Gross, and Karim Adeyemi in Lisbon had already given the German side a strong advantage.
Dortmund dominated proceedings and could have taken the lead in the second half when Rui Silva fouled Adeyemi in the box. However, Silva redeemed himself by saving Guirassy's penalty. The hosts, last season's beaten finalists, will now face either Aston Villa or Lille in the next round.
Sporting, who lead the Portuguese league by two points, were a major disappointment and never looked likely to overturn the deficit. Missing top scorer Viktor Gyokeres and winger Francisco Trincao, they struggled to create chances. Dortmund, fielding an unchanged lineup, controlled the game from start to finish.
Marcel Sabitzer tested Silva with a powerful strike, while Guirassy and Adeyemi also forced saves. Giovanni Reyna struck the post late on, and Emre Can had a goal disallowed for offside. Despite the win, new Dortmund boss Niko Kovac remains winless at home after two games.
