A source said: "They've been fighting for a while, but now they're done. They decided to finally split. It's sad."
Earlier this year, Jon completed rehab for alcohol abuse ahead of the final season of critically lauded Mad Men.
After finishing a 30-day stint in a treatment centre, Hamm credited Westfeldt, 45, with helping him overcome his addiction.
The actor's representative said at the time: "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."
The TV hunk and his partner previously spoke of their decision not to have children together.
In 2011, Hamm said: "I have a tremendous amount of respect for my friends who do balance this particular career with raising a kid, 'cause it's a tricky environment to bring them into... So not for us just yet."
- Bang! Showbiz