"With great sadness, we have decided to separate, after 18 years of love and shared history," the couple tells PEOPLE in a joint statement. "We will continue to be supportive of each other in every way possible moving forward."
Having been plagued with split rumors for months, Hamm, 44, and Wesfeldt, 45, were last seen publicly together on July 22 in New York at the premiere after-party for Netflix's Wet Hot American Summer: First Day of Camp, where they were "canoodling" with each other, according to reports.
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Earlier this year, the Mad Men star completed rehab for alcohol abuse and credited Westfeldt for supporting him throughout his treatment. In April, the couple even released a statement denying reports they had broken up.
Hamm told PEOPLE last year that he felt lucky to have Westfeldt, an actress and screenwriter, in his life. "Our lives are entwined," the actor said. "It's a long time [together]. I'm definitely the guy that repeats his stories! Mostly it's just paying attention and keeping engaged and not succumbing to inertia or going through the motions."
The two began dating in 1997 and starred together in the 2011 film Friends with Kids.
With reporting by AILI NAHAS