'Seinfeld' Actor -- Checks Into Rehab for Pill Addiction

John Pinette -- best known for his role as the fat guy who got mugged in the "Seinfeld" finale -- has checked himself into rehab after becoming addicted to prescription medication ... TMZ has learned.

A rep for the venue in Canada where John was set to perform in next month says Pinette had to postpone the gig because he was checking into a rehab facility for his addiction.

The rep says John -- who also had his own Comedy Central special in 2011 -- underwent surgery earlier this year and got hooked to the prescription medication he was taking.

John has since tweeted to his fans about the situation -- saying, "Thanks to everyone for your concern and well wishes.. Want to apologize for having missed shows last weekend, but I hope you understand!"

In case you're not a "Seinfeld" fan (WHAT'S WRONG WITH YOU?!?) ... Here's his famous scene from the super disappointing finale.

At least the reunion on "Curb Your Enthusiasm" kinda-sorta-not-really made up for it.