Actress Lizzy Caplan has brushed off rumours suggesting she is dating X-Men hunk James Marsden, insisting they are simply "best friends".

The Masters of Sex star appeared alongside Marsden in 2012 comedy Bachelorette and they recently sparked speculation their relationship had turned romantic after they were spotted spending time together on numerous occasions.

However, Caplan is adamant the claims are not true, telling, "He's just one of my best friends. He's one of my dearest friends."

The actress previously dated Friends star Matthew Perry for six years before their split in early 2012, while Marsden separated from his wife of 11 years, Lisa Linde, in 2011.