The 30-year-old is said to have given birth to their daughter on Friday, with E! confirming the happy news.
A source close to the couple told them: "Their whole family was around to support them.
"Evan was very emotional and loves Ashlee so much. He did not leave her side and wanted to go through this with her every step of the way.
"He can't wait to be a dad. He would do anything for Ashlee and now this baby girl. Ashlee is doing great and can't wait to get home to be with her family."
Ashlee already has six-year-old son Bronx from her marriage to Fall Out Boy star Pete Wentz, but the little girl is 26-year-old Evan's first child.
She and Evan, son of legendary singer Diana, tied the knot last August, and he recently shared some adorable photos from their big day.
Alongside a snap of him with Ashlee and Bronz, 90210 actor Evan wrote: "Such a beautiful wedding. Me and my amazing wife @ashleesimpsonross. At my mother's house in Greenwich Connecticut! And now I can't wait to meet my little baby girl [sic]."
It looks like their family is finally complete! Congratulations to Ashlee, Evan and Bronx!