The former reality star and singer gave birth to baby girl Jagger Snow on Thursday night. Simpson is already mother to Bronx Mowgli, 6, from her previous marriage to Pete Wentz.
"We are overjoyed about the birth of our precious baby girl," the couple posted on Instagram.
Simpson, 30, and Ross, 26, wed last August at the home of Ross' mother, Diana Ross. Jagger Snow is the third grandchild for the Supremes alum.
Check out Simpson's cute Instagram announcement below.