Justin Bieber wet his whistle, solo, in an unexpected hot spot ... a North Hollywood gay bar.
We're told Justin walked into Club Cobra on Saturday at around 9 PM with only one bodyguard and ordered a beer.
We spoke to the club's bouncer, who got a pic with JB ... he tells us he doesn't think Justin knew it was a gay bar. No one was there, except for the bartender and staff.
The bar -- which has events like Wayback Wednesdays and Forbidden Fridayz -- doesn't get hoppin' till 10:30ish on weekends ... the dancers come on at 11. Justin left around 10 PM, so it was likely a 1 OAK pre-game pit stop.