Nicki Minaj has a pissed-off Barb on her hands -- one of her avid fans claims Nicki publicly humiliated him by calling him a mentally unstable stalker and ... so, now he's suing.

The New York rapper was named as a defendant in the suit filed by plaintiff Tameer Peak in L.A. Superior Court – and the allegations paint Nicki as a full-blown bully.

According to the documents, obtained by TMZ, Tameer refers to himself as one of Nicki's most ardent followers for the past 15 years, and claims he helped her career by promoting her 2023 album, "Pink Friday 2."

But Tameer says things soured in 2017, when, according to the suit ... Nicki went through her "Nicki hate train" phase, leading her to accuse fans -- like Tameer -- of being paid moles sent to sabotage her.

He claims Nicki publicly ridiculed him, inviting him to her hotel on one occasion to film and embarrass him in front of 100,000 viewers on Instagram Live.

The docs say Nicki also mistreated him during a Super Bowl weekend event in 2020 when her husband, Kenneth Petty, battered him. Peak says Nicki later sent him a DM referring to the incident.

Then in 2024, he says he got messages from others that Nicki wanted him to "stop saying dumb s--t," hurting his reputation with her fans.

Tameer claims Nicki also targeted him on Twitter (now X) during several Stationhead sessions, where she would interact live with fans.

In one session, Nicki allegedly wrote, "It's clear they're not necessarily, you know, a full basket umm at the picnic." Peak took that to mean he was mentally unstable or intellectually deficient.

In another, he says Nicki accused him of being "one sandwich short of a picnic" and receiving a check from the government, implying he was on welfare.

Nicki allegedly went on to label Tameer a "stalker" who impersonated members of her team.

As a result, he's suing Nicki for $5 million in damages for reputational harm, emotional distress and financial loss. We've reached out to Nicki's rep for her side of this Barbz saga.