The actor, who played Don Draper in the advertising drama, has confirmed the long-time romance has come to an end.
In a statement to People, the couple said: "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."
Jon and Jennifer, a director and writer, were hit by split rumours back in April, forcing his rep to deny any plans to part ways. The story came after Jon admitted he'd entered rehab for alcohol addiction treatment.
In March, a rep for the star said: "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."
Jon and Jennifer have been together since 1997.