The "Trainwreck" star revealed the news on Instagram Monday, sharing a sweet photo of herself holding the couple's newborn son while Fischer kissed Schumer's head.
"10:55 pm last night. Our royal baby was born," the 37-year-old actress-comedian wrote.
Moments before Schumer announced the news, she posted another photo of herself posing on the Metropolitan Museum of Art's steps ahead of the annual fashion gala in New York City, which takes place every first Monday of May.
"Met look this year. Last night on the way to the hospital," Schumer — in black sweats, a gray top paired with a cardigan — teased.
Schumer and Fischer tied the knot last February in a surprise beachside wedding.