Richie, 28, confirmed the engagement during a taping of CBS' "Late Show With David Letterman." The show will air Friday at 11:35 p.m., The Associated Press reports.
Letterman asked, "You're not married yet, is that correct?" Richie replied, "We're not married." Letterman then asked if she and Madden planned to get married.
Richie said, "We actually recently are engaged, yes." Madden, 30, fronts the band Good Charlotte.
The couple have a daughter, Harlow Winter Kate, 2, and a son, Sparrow Midnight, 5 months.
Richie has a new clothing line, Winter Kate, and shoe and accessories line, House of Harlow.
She and Paris Hilton co-starred on TV's "The Simple Life."