Frank "Knuckles" Walker -- who joined the band in 2002 but was cut loose back in May -- says the band is promoting performances as if he's still with the band. Frank also alleges the band never paid him his fair share of royalties.

In the suit -- first reported by the NY Daily News -- Frank says, by promoting appearances as if he's still a member, the band's using his likeness without permission. Frank adds that his legacy could be damaged if The Roots continue promoting gigs without admitting to fans he's no longer part of the group.

He calls the act despicable but doesn't elaborate on how the band shorted him. Frank's suing to get his cut and for The Roots to stop using pics of him to promote itself.

We've reached out to The Roots, so far no word back.