There are around 500 harbour seals and 200 of the larger grey seals in the Thames Estuary, according to the Zoological Society of London (ZSL) said.
ZSL's conservation scientist Joanna Barker said:
We knew there were a lot of seals in the Thames but 708 is pretty incredible.
In previous results there's been a good few hundred in the Thames, but it's great to have a figure we can use as a baseline.
Now we know the numbers and where they are, it can help with conservation.
It's a really good indicator because the seals are the top predators in the marine food chain, and it shows that the marine environment is relatively good and is producing enough food for the seals to eat.
More top news
A police officer who spent six days crawling the London Marathon in a gorilla costume has finally completed the 26-mile route.
The victim of a "violent and sustained attack" on the top deck of a London bus has been named as Archie Sheppard.
Transport for London has made the badges and accompanying cards permanently available after a successful trial in autumn last year.