Ex-NFL player Sergio Brown was spotted in Tulum on Sunday ... dancing and partying his face off -- this despite an ongoing police investigation into the death of his mother.
Brown -- wearing red Nike swim trunks -- didn't seem to have a care in the world as he swayed shirtless with patrons on the dance floor at a popular club in the Mexican city.
One witness tells TMZ Sports ... the former Indianapolis Colts defensive back was at the venue for several hours -- chatting it up with people and buying alcohol for others, before bolting.