He will be on duty at a base on an island off the coast of Wales and is expected to celebrate his birthday privately.
A low-key day may be just what the prince wants after his blow-out April wedding to Kate Middleton.
He is also getting ready for a trip to Canada and the U.S. next month. It will be his first official overseas trip as a married man.
William's grandfather, Prince Philip, recently celebrated his 90th birthday with a full day of official engagements.
The Associated Press contributed to this report.