Simpson is the younger singer of actress and recording artist Jessica Simpson. Wentz is the bassist for the pop group Fall Out Boy.
The couple reportedly started dating in 2006.
"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 the online posting. "Yes, we are thrilled to share that we are happily engaged. Thank you for all of your support and well wishes -- it means the world to us. We consider this to be a very private matter, and we wanted to be the first to tell you and to hear it straight from us."
Simpson's father, Joe, told People.com he is "totally happy" and "so excited to have Pete as part of (our) family."
"We are blessed," he said.