Kenny Lofton -- Wrote For Ruben Studdard ... On Singer's #1 Album

Did you know ... MLB legend Kenny Lofton WRITES R&B MUSIC in his spare time ... and wrote one of the songs on Ruben Studdard's biggest album?!!

It's true ... so says the Velvet Teddybear himself ... who told TMZ Sports the former Cleveland Indians superstar (and likely future Hall of Famer) did some serious writing work on a song called, "What If" from his 2003 album "Soulful."

FYI -- Soulful was a smash hit for Studdard ... hitting #1 on the U.S. Billboard music charts and selling 1.8 million copies.

Studdard tells us ... Lofton had been writing music since he was in college with one of his good friends -- who turned out out to be a 6-time Grammy winner!