Guernsey’s Coastguard said the Commodore Goodwill reported a capsized vessel, with someone in the water around five miles east of the island around 11:45.
The ship then launched its rescue boat, while Alderney’s lifeboat was also launched to come and help.
Shortly after 12:00, a man was recovered from the water and transferred to the ship.
He was said to be “safe and well”.
Alderney’s lifeboat arrived around 12:30 and transferred the man to Alderney.
He was met by paramedics and Harbour Officers at Braye Harbour, while the Commodore Goodwill continued its journey to Portsmouth.