The Fashion Star mentor welcomed her second child with fiance Eric Johnson, a baby boy, on Sunday.
The boy, who the couple has named Ace Knute Johnson, was born in Los Angeles via a scheduled C-section, Us Weekly reported.
"Ace Knute (pronounced Ka-nute) Johnson has arrived! Mom and baby are doing great," Simpson's representative told Us. "Jessica, Eric and Maxwell are thrilled to welcome the new addition to their family."
Simpson had become pregnant with Ace only seven months after Maxwell's birth and announced the pregnancy in a Christmas Day tweet in which she had posted a holiday photo of Maxwell sitting in the sand with "BIG SIS" written in the sand in front of her.
Simpson previously admitted the new pregnancy wasn't planned, as she was "extremely shocked" when she first found out she was expecting again.