Roy Rosselló -- who was part of the uber-popular boy band from the '80s -- is the subject of a new docuseries that's being aired by Peacock ... titled, "Menendez + Menudo: Boys Betrayed," which teases a big bombshell that Roy says he's ready to share with the world.

He claims Jose Menendez drugged and sexually assaulted him before Menendez was brutally killed alongside his wife by their sons, Lyle and Erik.

Unclear how exactly Roy claims to have known Jose during the time of the alleged incident -- but it's not outside the realm of possibility ... remember, Menendez worked as an executive in the music biz during the '80s and '90s -- and was tight with the Latino community.

Rossello's story is scheduled to stream May 2 ... but from the trailers/promos released this week, there's no doubt he's leveling a serious allegation that seems to jibe with what the Menendez Bros. also claimed about their dad in the '90s ... namely, that he'd allegedly sexually abused them for years during their childhood.

You'll recall ... the lynchpin of the brothers' defense essentially boiled down to telling the jury they lived in fear from the abuse and only killed their parents after claiming their dad threatened them when they said they would expose what he'd done.

It's unclear for now if Rossello's new assertion -- which seems to be the first time anyone has publicly backed the notion that Jose Menendez might've sexually abused minors -- will have any impact on the Menendez brothers' case, as their lawyers have tried for years to secure a new trial with several unsuccessful appeals.