Lamar Odom -- NOT In Rehab

Lamar Odom has NOT checked himself in to rehab ... sources connected to the NBAer tell TMZ.

There are reports out that Lamar checked himself in ... we've been making calls for the last day and just got what we're told is the definitive word ... he's not there.

Lamar has gone off the radar with the Kardashian family for the last few days. Khloe -- who has made it clear she will end the marriage if he doesn't get help -- has heard nothing from him.