One person was taken to hospital after suffering a head injury on Flat Holm Island.
The RNLI at Barry Dock said its all-weather lifeboat was sent out to deal with a medical evacuation on Wednesday evening.
The casualty, who was described as conscious but dazed, was taken off the island as a precuation to meet an ambulance at the Cardiff Bay Barrage.
A spokesperson said: "On arrival in Cardiff, the casualty was met by a parent and handed over into their care and onwards to accident and emergency."
The RNLI also thanked the official warden at Flat Holm for helping with the coastguard and the lifeboat as well as the crew at the barrage for a "swift lock in".