The newborn boy, named Ace Knute Johnson, is Simpson's second child with her fiance, Eric Johnson, a former NFL player. Ace Knute joins his one-year-old big sister, Maxwell Drew.
Simpson's father, Joe, expressed his excitement over Twitter.
"Very blessed to celebrate the birth of my new grandson," he wrote.
Simpson confirmed her pregnancy over Twitter on Christmas Day last year, sharing a picture of her daughter, Maxwell, sitting above the words, "big sis," written in the sand.
Merry Christmas from my family to yours! pic.twitter.com/EjlCoYbq— Jessica Simpson (@JessicaSimpson) December 25, 2012