The couple, who got married at the end of August last year, welcomed their daughter Thursday night, according to E! News, which first had the story.
Ross, 27, who's singer Diana Ross' son, "was very emotional and loves Ashlee so much," a source told E!. "He did not leave her side and wanted to go through this with her every step of the way." The outlet said she was surrounded by family as she brought her second child into the world.
Six-year-old Bronx, 30-year-old Simpson's son with ex Pete Wentz, takes on the role of big brother.
The little lady will also have a couple of beach buddies in cousins Maxwell and Ace, daughter and son of sis Jessica Simpson and her husband, Eric Johnson.
The child's name and other details were not immediately available.