The rep told Us Weekly they named the baby Gunner Stone.
"Heidi and Spencer are proud parents of a healthy baby boy as of this afternoon," the rep said.
Montag confirmed the news on Twitter.
She gave birth to Gunner at 3:06 p.m. He weighed 6 pounds, 12 ounces and was 19 inches long.
"We are so blessed to have a beautiful healthy baby boy," Montag told Us Weekly. "It was the hardest and most rewarding experience."
Montag announced her pregnancy in April, and confirmed the next month that she and Pratt are expecting a baby boy. She said in an interview with Us Weekly that it was "in [her] heart" to have a son.
"I thought Spencer really used to want a boy, and then a few months ago we had a conversation where he said, 'Let's just make sure it's healthy. Boy or girl, I don't care,'" the star recalled.
"I hope to have a little girl one day, but for my first child, I really wanted a son," she shared. "I thought it'd be a more natural bond for Spencer since he and his dad are so close."
Annie Martin contributed to this report.