WILL SMITH is hiding a shocking secret arrest – and in a blockbuster exclusive, The ENQUIRER has uncovered his never-before-seen mug shot!

The “Men in Black” star looked like a young thug hanging out with his homeboys when he ran afoul of the law for allegedly being involved in a brutal beat-down that left one man nearly blind, sources say. The charges were later dismissed, but the case was “a huge embarrassment” for Smith, said the insider.

Cops hauled Smith off to jail where he spent time behind bars, and The ENQUIRER has uncovered dramatic details of the embarrassing episode.

“Will has tried desperately to sweep this whole thing under the rug, especially since he has such a squeaky-clean reputation,” said an insider. “The last thing he needs is for this police photo to be published.”

The vicious assault occurred in Smith’s hometown of Philadelphia in March 1989. Just days earlier, the then-20-year-old had won the first Grammy ever awarded for rap as half of the chart-topping duo DJ Jazzy Jeff and the Fresh Prince.

According to police, record promoter William Hendricks got into a quarrel with Smith, who then allegedly instructed his bodyguard, Charles “Charlie Mack” Alston, to “get” Hendricks.

Hendricks, then 38, suffered a fracture of his left eye orbit and required six stitches after the attack. The ENQUIRER tracked down Hendricks, but even after all these years, he still didn’t want to talk about it.

Cops arrested Smith and charged him with aggravated assault, recklessly endangering another person, simple assault and criminal conspiracy.

“Will had to spend a night in a cell at the West Philadelphia police station with other inmates waking him up all night long and asking for his autograph,” the insider noted.

“It was the worst night of his life. He wants to forget it ever happened.”

Soon after, SMITH, 44, soared to stardom in TV’s “The Fresh Prince of Bel-Air” and later cashed in with the big-screen blockbusters “Independence Day,” “Bad Boys,” “Hancock” and “I Am Legend.” He also earned Academy Award nominations for his roles in the 2001 biopic “Ali” and the 2006 drama “The Pursuit of Happyness.”

The rapper-turned-actor has rarely spoken about his arrest, but in a 2005 interview, he insisted: “It wasn’t my fault. That’s what they all say! But it wasn’t my fault. This is back in Philly. One of my buddies beat up this dude and I was there.

“That’s my story and I’m sticking to it.”