The couple will tie the knot after three years of dating, Richie announced on "The Late Show With David Letterman" Monday.
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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.