Marsan was first elected in the Robert-Baldwin riding in 1994 and has held the seat ever since.
"After 20 years in politics I decided, after consulting my wife ... and my two sons, that it was time to step down and to give the opportunity for someone else to work," he said.
Marsan said he looks forward to working with the new candidate in order to ensure the riding remains a Liberal stronghold.
St-Amand, who was first elected in 2008, said she has decided to pull out because of health reasons. She suffered an illness several months ago, and St-Arnaud said she is having trouble sleeping and keeping up with her work.
For now, St-Amand said she plans to take some time to rest and recover.
Marsan's legacy marred by controversial letter
Veteran politician Marsan sparked controversy last fall over a letter he sent to his constituents.
The letter, signed by Marsan, was distributed at a synagogue in Dollard-des-Ormeaux.
The letter reminded members of the West Island Sephardic Jewish Association that when Quebec Liberals were in power, the Jewish community had "profited from generous contributions."
The controversial letter prompted an investigation by Quebec's chief electoral officer.
Marsan later apologized to the national assembly.