Simpson gave birth to a daughter named Maxwell Drew Johnson in Los Angeles on Tuesday, said publicist Lauren Auslander.
Maxwell weighed 9 lbs. 13 ounces.
Since announcing her pregnancy last Halloween over Twitter, Simpson posed nude for the cover of Elle magazine, shared her cravings like salted cantaloupe and pop tarts and joked about her excess of amniotic fluid on "Jimmy Kimmel Live."
The name is a tribute to the couple's families: Maxwell is Johnson's middle name and the maiden name of his mother, and Drew is Simpson's mother's maiden name.
"We are so grateful for all of the love, support and prayers we have received," Simpson said in a statement on her website. "This has been the greatest experience of our lives!!"
Simpson had come in for some ribbing about her seemingly endless pregnancy. She kept up public appearances, including on NBC's "Fashion Star," and used Twitter to send pregnancy updates, such as her dream of wearing a leopard caftan in the hospital.
Simpson addressed the rumor mill herself in April, tweeting: "To everyone who keeps congratulating me on the birth of my baby girl ... I'm still pregnant!"