Vic Mensahas a bad trip - back from Africa, that is - because US customs agents have had him remanded in custody for allegedly returning with a lot of mushrooms ... TMZ has learned.

The rapper was remanded in custody Saturday at Dulles International Airport outside DC ... where he had just returned from Ghana. According to our law enforcement sources, Vic was pulled aside for a secondary search and that was when agents found what they believe are mushrooms.

We are told that the US authorities are still testing the substance they allegedly found to confirm that this is what they think it is.

Vic was charged with aggravated possession of drugs. Fun facts: Psilocybin (the scientific name for fungi) remains a Schedule I drug under federal law - although there is now a push in some states to legalize it ... and they have already been decriminalized in some societies.

Vic had been hanging out in Ghana with Chance the Rapper and many people in high places. The duo posed for photos with the president Nana Akufo-Addo.

The Chicago-born and raised artists had been traveling around Africa for a few weeks to promote tourism.

We've reached out to Vic's representatives, but there's nothing left yet.