"Got myself a fiancé just before new year," he wrote on an Instagram photo of him and Seaborn. "We are very happy and in love, and our cats are chuffed as well xx."
Sheeran, 26, and accountant Seaborn, 25, were pupils at the same school in Suffolk, eastern England and began dating in 2015.
The ginger-haired Grammy winner has enjoyed a string of hits since his debut album + (Plus) topped the UK charts in 2011 and reached No. 5 in the United States.
His single "Shape of You" was 2017's biggest selling single in the UK, according to the Official Charts Company, and "Perfect", featuring Beyonce, was Britain's Christmas No. 1 last year.
Seaborn, a keen field-hockey player, studied at Duke University in North Carolina and later worked in the Wall Street financial district of New York before transferring back to England.
A date for the wedding has yet to be announced.
(Reporting by Stephen Addison; editing by Alexander Smith)