Odell Beckham Jr. is rallying behind Chris Brown, who's been sitting in a UK jail cell since Thursday, taking to social media with a message of support -- "Free Breeeezyyy!"
Brown, 36, was locked up last week for an alleged incident going back to 2023 ... when the Grammy Award-winning performer allegedly smashed a liquor bottle over a music producer's head.
CB was charged with one count of grievous bodily harm ... and will remain in custody until his first appearance at London's Southwark Crown Court on June 13.