Prince William co-piloted a helicopter which rescued two people after a ship was hit by an "enormous wave" in the Irish Sea.
One person died and another five are still missing after the 81-metre cargo carrier sank some 10 miles west of the Lleyn peninsula in north Wales.
The Duke of Cambridge was called into action after the Swanland ship issued a mayday call at around 2am when its hull cracked.
He has now returned to his base at RAF Valley in Anglesey, Wales, the Ministry of Defence said.
Two members of the Russian crew of the Cook Islands-registered vessel were pulled from the water soon after the alarm was raised.
They were airlifted to safety by William's Sea King helicopter and were later taken to hospital in Bangor for precautionary checks.
Another crew member was found in the water later in the morning but has been confirmed dead, said the Maritime and Coastguard Agency.
Two RAF Valley search and rescue helicopters were deployed to assist with the rescue operation.
A spokesman for the Ministry of Defence said: "The Duke of Cambridge was a co-pilot on one of the aircraft involved. He is now back at base."
William is understood to have been involved in the operation over several hours.