"With great sadness, we have decided to separate, after 18 years of love and shared history. We will continue to be supportive of each other in every way possible moving forward," the couple tells People magazine in a joint statement.
Hamm, 44, and Westfeldt, 45, have been together since 1997. They even worked together on a recent film "Friends with Kids."
But earlier this year, rumors about the couple's split had been bubbling to the surface. Nevertheless, Hamm had said how supporting Westfeldt had been earlier this year during his stay in rehab.
"With the support of his longtime partner Jennifer Westfeldt, Jon Hamm recently completed treatment for his struggle with alcohol addiction. They have asked for privacy and sensitivity going forward," his publicist said in March.
A rep for Hamm could not immediately be reached for comment.