The reigning champions wasted little time asserting their superiority, with Ballon d’Or winner Ousmane Dembélé opening the scoring in the 17th minute after Nuno Mendes’ effort struck the post.
Dembélé doubled PSG’s advantage six minutes later, heading home from a cross involving Dro Fernández to put the hosts firmly in control.
Torres then joined the scoring in the 31st minute, finishing at the near post after being picked out by Dembélé. The Spanish forward struck again shortly after half-time, making it 4–0 in the 47th minute.
The former Barcelona forward completed his hat-trick in the 57th minute, capping a remarkable individual performance on his first Champions League appearance for PSG.
Slovan Bratislava briefly reduced the deficit through Suleiman Camara in the 58th minute, but the visitors' response proved little more than a consolation.
Fabián Ruiz restored PSG's five-goal advantage in the closing stages, completing a commanding 6–1 victory for Luis Enrique's side.
• Paris Saint-Germain: Ousmane Dembélé 17', 23', Ferran Torres 31', 47', 57', Fabián Ruiz 88'
• Slovan Bratislava: Suleiman Camara 58'
The result represents an ideal start to PSG's Champions League title defence after a difficult opening to their domestic campaign. PSG had entered the European opener without a win in their first three Ligue 1 matches, including a 2–1 home defeat to Monaco last Friday.
For Torres, the hat-trick provides an immediate statement of intent following his arrival at the French champions. His three goals, combined with Dembélé's brace, underlined PSG's attacking strength as they launched their latest bid for European glory.
PSG will next face Manchester City in the Champions League on October 14, while their immediate domestic assignment is an away trip to Brest in Ligue 1 on September 13.