The singer/actress and her actor beau announced the name on their Instagram.com accounts following the birth of Jagger on Thursday (30Jul15).
A post on the social networking site on Friday (31Jul15) states, "We are overjoyed about the birth of our precious baby girl, Jagger Snow Ross. July 30, 2015. - Ashlee & Evan".
The couple wed in September last year (14) at the home of Ross' mother, music icon Diana Ross. Simpson has a son - Bronx Mowgli - from her previous marriage to Fall Out Boy guitarist Pete Wentz.