He will face defending champion Stefanos Tsitsipas for the title.
No.2 seed Tsitsipas was in impressive form as he dispatched Alexander Bublik in the day's first match, running out 7-5 6-3 victor to reach the final of the tournament for a second year in a row.
The Greek continued his run of not dropping a set at the competition since a first-round defeat to Nicolas Mahut two years ago, racking up a 14th successive win in Marseille.
Tsitsipas was particularly devastating on his own serve in the second set, dropping just a single point, and he was delighted to be able to keep Bublik under control.
"I tried to stay in the match as much as possible and play each point individually," he said. "I know he can be [Bublik] quite unexpected.
"You don't know what to expect. I didn't know what to expect from today's opponent. I am really happy with myself that I found a balance and I didn't panic when things got tight.
"That is a great attitude that I put out on the court. It keeps getting better and better."
Auger-Aliassime will be hoping to go one better than he did in Rotterdam last week, having lost in the final of that event to Gael Monfils.
The Canadian did not have it all his own way against Simon, who eliminated Daniil Medvedev in the last round, but Auger-Aliassime showed admirable resilience to claim a double break in the first set after losing his own serve early on.
Simon took the second set to a tie-break, but he rarely appeared to be in the conversation at that stage, with Auger-Aliassime eventually winning 7-5 7-6 (7-2).