Melvin Upton's wife likes "B.J." ... so now his name is B.J.
The ex-MLB star used to go by the initials -- it was an homage to his father, whose nickname was "Bossman" ... and therefore made him "Bossman Jr."
But, B.J. switched his name back to his birth name Melvin in 2015 -- 'cause it was his real name and what almost everyone close to him called him.
But now, Upton is back to B.J. ... and tells TMZ Sports it really wasn't his decision.
"My wife. She said Melvin didn't sound right."
She's kinda got a point ... Melvin Upton Jr. is a mouthful to say for a guy who's retired and is now trying to make a name for himself as a baseball analyst on television.
Plus ... when he was "B.J." -- the dude was a superstar ... hitting around 20 home runs and stealing around 40 bags with ease during seasons.
AND, as they say -- happy wife, happy life ... right, B.J.??
There's more ... Mr. Upton also tells us why he's retired FOR GOOD and won't attempt a comeback (his back's all jacked up).
Plus, he makes the case for Barry Bonds going to Cooperstown.