The 39-year-old - known for playing Anya on Buffy - has been cast as Winnie, a New Jersey baker who is Hank's (Mark Feuerstein) ex-girlfriend, TVLine reports.
Caulfield first rose to fame in Beverly Hills, 90210 and has recently appeared on TNT's Leverage and ABC's Once Upon A Time.
She will appear in the final episodes of Royal Pains's fourth season. The series wraps its summer run on September 19 and resumes in late 2012.
Royal Pains continues on Wednesdays at 9/8c on USA. In the UK, the show airs on the Universal Channel.
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Watch a promo for tonight's episode of Royal Pains below: