Mike Tyson and Ric Flair were on cloud nine Tuesday night ... 'cause the two sports legends smoked doinks together after attending a cannabis conference in Chicago -- and judging by the vid, it was some GOOD stuff!!

The Nature Boy posted a clip alongside Iron Mike ... lighting up blunts on camera, smoking and giggling together out in the streets.

"Smokin' That Ric Flair Drip All Night Long With @miketyson! WOOOOO!" Flair captioned the video.

Tyson and Flair became partners in Mike's "Tyson 2.0" business earlier in the year -- and the two collaborated on Naitch's "Ric Flair Drip" cannabis line, according to Benzinga.

Flair and Tyson smoked multiple blunts on stage at the conference, according to Benzinga ... where Mike and Ric touted the benefits of smoking marijuana over using drugs like Xanax for pain relief.

Flair said during the event that smoking pot was a much better way to manage the pain he was experiencing due to the stress the wrestling industry put on his body.

This isn't the first time the two legends blew down ... Mike and Ric smoked with Rick Ross at a Benzinga Cannabis event in April -- after the boxer famously beat up a man on an airplane.