"We know there has been a lot of speculation recently about Pete and I, and we wanted our fans to be the first to know, because you guys are the best," Simpson said in a statement posted Wednesday on People magazine's Web site. "Yes, we are thrilled to share that we are happily engaged."
Simpson's manager-father, Joe Simpson, told the magazine he's "totally happy" and "so excited to have Pete as part of (our) family."
The couple began dating in fall 2006, according to People. Wentz, 28, is bassist for Fall Out Boy. Simpson, 23, a singer, songwriter and actress, is the younger sister of pop star Jessica Simpson. Ashlee Simpson's new album, "Bittersweet World," is set for release on April 22.
A phone message left Wednesday for Joe Simpson by The Associated Press was not immediately returned.
