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
Three men from East London have been jailed for stealing vital life-saving equipment worth over £100,000 from fire stations.
A 19 year-old from South East London has pleaded guilty after he soiled himself and revealed where he had hidden packets of heroin.
Max Whitlock was greeted by hundreds of fans as he returned to his gym in Essex for the first time since winning double gold in Rio.