The RSPCA said it believed the adult grey seal swam inland along the River Glaven in Norfolk on New Year's Eve.
It then managed to make its way into a woman's garden in Blakeney, just off a coastal road, through the fence. It was returned to the sea on New Year's Day.
The RSPCA said: "The lady certainly had a shock when she spotted him lazing around in her shrubbery."
The seal was spotted by the holiday home owner in the afternoon.
Animal collection officer Naemi Kilbey said the animal was "in very good condition".
"Despite being well and truly lost, he luckily didn't have any obvious injuries or illness," she said.
"So we loaded him up in the van and took him straight down to the beach at Blakeney to release him.
"It was so lovely to start the new year watching him swim back out to sea where he belongs."
The RSPCA said it was "not uncommon" for seals to venture up rivers away from the sea looking for food.
It said they normally managed to find their way back.
"But it looks like this guy's internal navigation system got a bit frazzled and he ended up a little lost," Ms Kilbey added.
The RSPCA advises members of the public to keep a distance from any seals on land and not to interfere unless necessary.
A seal was found in the back garden of a holiday home, 2km (1.2 miles) away from the sea.