Former NFL Player Keith Browner Sr. Dead at 63

Keith Browner Sr., a standout football player at the University of Southern California who went on to play 5 seasons in the NFL, is dead ... TMZ Sports has learned.

Keith died Tuesday morning in San Leandro, CA ... according to his son, Keith Browner Jr.

We're told Keith's death was unexpected and sudden ... and the family is shocked.

The cause of death is unclear, but we're told it appears he suffered a heart attack.

Keith Jr. tells us he talked to his father Monday night and he was having stomach problems, was throwing up, felt tired and said he would go to the hospital Tuesday morning.

We're told Keith Sr. was up Tuesday morning getting ready to go to the hospital to get checked out when he…