Alistair Forrest, 40, was out with a local kayaking club when the seal climbed on board.
The engineer was around four miles east of the Forth Road Bridge, Edinburgh, when the grey seal began circling his boat.
It is thought to have weighed between 90kg and 100kg as it forced the vessel to go halfway underwater.
Despite being “elated” with the encounter, Alistair, from Armadale, West Lothian, admitted to being apprehensive when the heavy mammal made its move.
He said: “I’ve seen plenty of seals in the Forth before but they’ve never got this close to me.
“I was, initially, a wee bit apprehensive because it’s such a heavy seal but once it got on board I was elated to be up so close.
“The rest of the club went ahead without me but I stopped about three times to take pictures and film it. I couldn’t believe it.
“It was extremely heavy on the kayak. It must have weighed around 100kg because the entire back end of my kayak was beneath the water. “