John Stamos -- Checks Into Rehab

John Stamos is in rehab ... five weeks after he was arrested for DUI.

The 51-year-old actor is in a residential program for substance abuse. TMZ broke the story ... Stamos got busted in Beverly Hills on June 12.

Cops cuffed the former "Full House" star on the scene, but then rushed him to a hospital for treatment ... believing he was suffering from a serious medical condition.

People first reported Stamos' rehab stint.

He's scheduled to be in court for the DUI in September.