Post Malone sustained a rib injury in a stage fall Saturday night during his St. Louis show after the rapper accidentally fell into an open trap door.

Midway through the concert, during his performance of “Circles,” Malone unknowingly walked toward where the stage door — used to shuttle equipment to and from the stage — was still open. Malone then partially plunged into the gap, slamming his ribs on the stage and, after a few moments of writhing in pain, called over for medical assistance.

After tending to his wounds backstage for roughly 15 minutes,...