T.I. played a key role in talking a suicidal man off the ledge of a 22-story building in Atlanta today ... TMZ has learned.

It all went down earlier this afternoon -- when cops responded to a 911 call about a man who was threatening to jump off a building in downtown Atlanta ... the same building where the Ryan Cameron Radio Show is based.

We're told T.I. heard about the jumper on the radio -- and called Ryan Cameron to see if there was anything he could do to help out with the situation.

Ryan Cameron tells us ... he told T.I. to roll over to the building, where the rapper met with police and recorded a heartfelt video message which cops then played for the man on the roof.

We're told after the man saw T.I.'s video he agreed to step off the ledge ... and met T.I. in the lobby, where the two were able to talk things out.

Ryan Cameron also tells us that cops informed the radio station that they will not be pressing charges against the man.

Calls to T.I.'s reps have not been returned.