City, who are still searching for their first Champions League final appearance despite their domestic dominance in recent years, were eliminated last term by Lyon at the quarter-final stage.
Porto were defeated by Krasnodar in the Champions League qualifying rounds last season, meaning the club entered the Europa League, where they were knocked out in the round of 32 by Bayer Leverkusen.
Olympiakos finished third in the Champions League group stages last season, behind Bayern Munich and Tottenham Hotspur, and defeated Arsenal in the Europa League round of 32 before going out against Wolves.
Meanwhile, Marseille were without European football last season. Here is everything you need to know:
Group stage draw
Seed 1: Porto
Seed 2: Manchester City
Seed 3: Olympiakos
Seed 4: Marseille
Group stage schedule
20/21 October: Group stage, Matchday 1
27/28 October: Group stage, Matchday 2
3/4 November: Group stage, Matchday 3
24/25 November: Group stage, Matchday 4
1/2 December: Group stage, Matchday 5
8/9 December: Group stage, Matchday 6