EXCLUSIVE

Matt Roberts, the former guitarist from 3 Doors Down has died of an apparent overdose ... TMZ has learned.

Matt's dad Darrell tells TMZ he and Matt were just outside Milwaukee for a charity event where Matt was scheduled to perform. Darrell says Matt had rehearsal until 1 AM Saturday and the two went back to their hotel with an adjoining room.

Darrell says he was woken up around 9 AM by detectives who informed Matt had passed away overnight. Darrell tells us prescription pills were involved ... but he believes the possible OD was accidental.

Matt left 3 Doors Down in 2012 for health issues concerning blood circulation in his body.

Darrell tells us Matt was deeply loved by his friends and family and the loss is devastating.