RSPCA officers rescued the animal from a farmer's land in Graig Penllyn near Cowbridge on Wednesday.
The charity believes the pup reached the field after swimming up the streams of the River Thaw.
After rehabilitation, it is hoped the pup can be released back into the sea in the near future.
RSPCA officer Gary Lucas said: "I've worked for the RSPCA for almost 30 years and this was my first ever seal rescue - and in quite amazing circumstances.
"As the crow flies, this amazing seal pup was almost seven miles from the sea and must have reached Graig Penllyn after travelling up the tiniest of streams possible.
"Thankfully, whilst the pup was a little thin and clearly dehydrated, he seems to be doing okay."

Locals in the Kathmandu Valley are being supported with food and water from The Brick Children School.
Headteacher Anita Shrestha said staff have been busy preparing food packages for 50 families.
The school, a charity project founded in Wrexham, has also been turned in to a refuge shelter.
Since the earthquake, the death toll has risen to over 7,000 and the Nepalese authorities have ruled out finding more survivors.

The 66-year-old Brisbane Broncos boss will speak to RFL officials before deciding whether to accept a position.
Steve McNamara's contract as England coach expired after the 2-1 series win over New Zealand last autumn.
Bennett told the Brisbane Courier-Mail: "We haven't finalised the role but I've been offered some opportunities. I have to find out how they see me involved."
Bennett is one of the game's most experienced coaches and was in charge of Brisbane for 20 years from their formation in 1988. He won six Grand Finals before moving to St George-Illawarra Dragons, where he won another NRL title in 2010.
In addition, he has had three spells with the Queensland State of Origin team and two spells in charge of the national team.
A passionate supporter of international rugby league, he was part of the New Zealand coaching team that won the World Cup in 2008, but was overlooked in favour of Mal Meninga when he applied for the Australia job again in December 2015.
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"Everything is on the table," said Bennett. "I wanted to coach Australia, but it's not available now. I wanted to make a contribution to international rugby league.
"About five years ago, the RFL first arrived at my doorstep. It is always about finding the right timing."
The RFL has refused to comment on any discussions with Bennett. It is carrying out a review of the England set-up, which will be concluded in February.
Bennett is due in England next month for Brisbane's World Club Series game at Wigan on 20 February.
England will play Australia, New Zealand and Scotland in the Four Nations tournament in November.
BBC Sport's Simon Stone
Wayne Bennett is one of the most respected coaches in rugby league and his appointment - even on a part-time basis - would be a huge coup for the RFL. But if, as anticipated, Bennett combined the job with his present role with Brisbane Broncos, it would raise an issue over how well he knew the Super League players he would be working with.
Many of England's top players, including Canterbury's James Graham and Sam Burgess, who recently returned to the sport after an ill-fated spell in rugby union, are based in Australia. But the majority remain in England and while Bennett will be able to watch virtually every Super League game on TV, he will be relying largely on the opinions of others when it comes to their personalities.
A dehydrated and underweight seal pup has been found in a field seven miles from the sea in Vale of Glamorgan.
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Families and children left homeless following the Nepal earthquake are being provided with help from north Wales.
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Former Australia coach Wayne Bennett has been offered a role with England by the Rugby Football League.