Life behind bars ain't too bad for Ronaldinho ... the soccer legend joined a prison league futbol tournament last week -- and reportedly won a pig for dominating the games!!
If ya missed it ... the 39-year-old Brazilian has been locked away in Paraguay prison after officials say he entered the country using a fake passport.
Dinho has been at the facility for about two weeks ... but he's clearly making the most of it -- because the dude joined a soccer league with other inmates and CRUSHED it.
Videos and pictures surfaced of the former Ballon d'Or winner out on the asphalt with the guys .