Radial nerve palsy commonly associated with humeral shaft fracture, at the time of injury (primary), after manipulation (secondary), or late onset (not related to injury or manipulation). For the latter, there are still few documented cases regarding late-onset radial nerve palsy. In this case report, we present a case of late-onset radial nerve palsy associated with humeral shaft fracture treated with coaptation splint, which became evident 5 weeks after treatment. Surgical exploration revealed that the radial nerve had become stretched and encapsulated in callus and fibrous tissue. Neurolysis was performed with subsequent recovery of the radial nerve 12 weeks after the surgery.