Richie, 28, revealed that she was engaged to Joel Madden Monday night while taping a Late Show with David Letterman appearance set to air Friday.
When Letterman asked if she and Madden, 30, were married, Richie responded, "We're not," but added, "We actually recently got engaged."
After congratulating her, Letterman joked, "A lot of people will have children and then they don't want to rush right into the marriage – you know, they wait 'til the kids get out of college, then they get married."
Richie and the Good Charlotte singer began dating in 2006 and have daughter Harlow, 2, and son Sparrow, 5 months. "When you have the kids," Letterman asked Richie, "everything changes, doesn't it?"
"It sounds cheesy to say, but it does," Richie said. "Your whole life changes and your priorities change 100 percent ... in the best way possible."