The rumours stem from a blog on The Daily, where it was claimed that Hamm "has been hitting the bars hard in recent weeks, chatting up pretty women who aren't his girlfriend".
Hamm's alleged dalliances apparently coincide with longtime partner Jennifer Westfeldt being away from home.
A spokesman for the Mad Men star confirmed that Westfeldt has been "doing a play at Vassar" for weeks, but insisted that no infidelity had taken place.
"There isn't anything scandalous going on… [Jon has just] been out with friends", Hamm's representative told Gossip Cop.
Westfeldt and Hamm have been together for 14 years, but have never married.
In March, Hamm declared that he loves Westfeldt because she has always been an "open book".
The couple were recently co-stars in the film Friends with Kids.
Watch a trailer for Friends with Kids below: