Justin Combs -- Back on the Party Train ... Hits Hollywood Nightclub

For the first time since that whole kettlebell thing ... Diddy's son Justin Combs was back out in public last night -- hitting up a Hollywood nightclub ... and TMZ Sports has the pics.

Combs and an entourage rolled through Playhouse ... and tried to keep things low key.

In fact, on his way out ... Justin covertly climbed into a waiting SUV to avoid the paps.

Justin's night out could be a sign that he's trying not to let the UCLA incident bring him down -- plus, he was there until roughly 3 in the morning ... so, we're guessing he was having a good time.

As we previously reported, there has been no official decision made about Justin's future with the team -- but several key players want him back.