Keyshawn Johnson UNLOADS -- Justin Bieber Is a Trickster Coward

Keyshawn Johnson tells TMZ ... Justin Bieber isn't just a coward -- he's a TRICKSTER ... who switched seats with his Ferrari passenger at the end of his wild ride through Calabasas so he could beat the rap for his reckless driving.

Keyshawn has finally broken his silence about the incident -- and it's clear ... the former NFL star is still extremely PISSED at Bieber.

"You got a 19-year-old kid ... feeling entitled ... speeding up and down the highway ... that's why all the neighbors is mad," Key says ... "You can't do that."

Johnson says he personally witnessed Bieber behind the wheel as he blazed through their private, gated Calabasas community earlier his week ... but says when he confronted the Ferrari at Bieber's home, Justin SWITCHED SEATS with his African-American passenger -- who he says was Lil Twist -- as they pulled into the garage.

"Look man, I know the difference between a black kid driving a car and white kid driving a car ... I got biracial kids."