Dempsey has joined the cast of New Line's romantic comedy "Valentine's Day," which is being directed by Garry Marshall.
The ensemble already includes Julia Roberts, Anne Hathaway, Jessica Alba, Jessica Biel, Jennifer Garner, Shirley MacLaine, Bradley Cooper, Ashton Kutcher, Topher Grace, Emma Roberts and Hector Elizondo.
"Valentine's" follows five slightly interconnecting stories playing out in Los Angeles on Valentine's Day.
Dempsey will play a married obstetrician dating Garner's character on the sly, with her unaware of his status.
Dempsey, one of the stars of ABC's "Grey's Anatomy," most recently appeared on the big screen in the 2008 romantic comedy "Made of Honor."