"Got myself a fiancé just before new year," he wrote on an Instagram photo of them together posted on Sunday morning.
"We are very happy and in love, and our cats are chuffed as well xx."
Sheeran, 26, and Seaborn, 25 went to high school together in Suffolk, and started dating in 2015.
Seaborn is an accountant and field-hockey champion who studied at Duke University in North Carolina.
A wedding date has not yet been announced.