The stars sparked speculation of a romance after they were spotted enjoying a night out together in New York City on Friday.
A source tells New York Post gossip column Page Six, "Geraghty had his arm around Ritter briefly as they chatted with their group of friends."
Ritter's spokesperson has now confirmed the relationship, telling the publication, "They are dating."
The pair is set to appear onscreen in upcoming romantic comedy, Audrey Hepburn's Neck, about a Japanese man obsessed with thin western women. They were due to start filming in Tokyo, Japan next month, but it is thought the shooting will be pushed back due to the recent earthquake and tsunami in the country.