6:01 PM PT -- Lamar's rep says he collapsed due to dehydration after an intense workout earlier in the day without the necessary fluids in his system. They also said it was very hot in the club, while adding ... he's "doing great."

Lamar Odom collapsed at his VIP booth at an L.A. nightclub early Sunday morning ... TMZ Sports has the video.

Odom was at Bootsy Bellows on the Sunset Strip when several staffers and patrons noticed the ex-NBA star collapsed to the ground around 2 AM. You can see security rush in to help and he seems to be conscious.

The person who shot the video tells TMZ Sports he saw Odom drinking for several hours before the incident.

The issue ... Odom has a serious history with substance abuse -- he almost died from a drug overdose at a brothel in 2015.

Odom has since gone to rehab and vowed to change his life -- but he's been partying a lot lately ... from strip clubs to nightclubs.