He was fishing from a rowing boat on the Bure, at Horning, when he ended up with a seal in his keep net.
The spot is 20 miles inland, meaning the animal had swum up the river from Great Yarmouth.
The angler, who has not been named, spotted the seal when it popped up and went for a fish he had caught.
It then followed the fish into the net, meaning the fisherman was left with both.
The seal was found to be “significantly underweight” (Image: Friends of Horsey Seals)
He contacted the volunteer rescue team at Friends of Horsey Seals, which came to collect the creature and take it for treatment.
A spokesman for the charity said the Boxing Day incident was “one of the strangest seal rescue stories of the year”.
Volunteer Sally Butler added: “I took the call just before 8am – the fisherman was in his little rowing boat, he’d caught a fish and he was trying to get it into his keep net and the seal went in behind the fish.”
The seal was found to be significantly underweight so was taken to the RSPCA’s base at East Winch, near King’s Lynn, for further assessment and treatment.
Ms Butler added: “The pup was only 12kg when, at this point, they need to be 30 to 45kg, so we took it to East Winch.
“As long as it starts feeding for itself, hopefully it will be OK.”
The pup, which has now been named Sunshine, was one of 49 calls received by the Friends of Horsey Seals on Friday.
It was said that the high number of calls was typical for the charity at this time of year as it was their busiest period.
Thousands of grey seals are born every year between November and January on the stretch of coast around Horsey, Sea Palling and Winterton-on-Sea near Great Yarmouth.
In some cases, young pups who are on the move can often cause concern amongst visitors, but generally do not need help.
It is not the first time seals have travelled inland through the rivers of the Broads. The creatures have been seen swimming on the Wensum in the centre of Norwich.