The couple will tie the knot after three years of dating, Richie announced on "The Late Show With David Letterman" Monday.
The two started dating in December 2006. They have two children, Harlow, 2, and Sparrow, 5 months.
Richie also debuted her engagement ring on the show.
The socialite-turned-designer's rep could not be reached by Usmagazine.com to comment.