The comedian hopes to raise over £200,000 for charity.
David Walliams started his big swim down the Thames river on Monday. He hopes to raise loads of money for Sport Relief charities.
He gets to take breaks along the way and aims to swim around 18 miles a day.
It's a tough challenge. Here's David receiving treatment after a long day's swim.
Swimming the length of the Thames can be pretty dangerous. Perhaps this man in the kayak is hoping to keep away curious swans? The river is also full of bacteria and David's already picked up a tummy bug - but he's vowed to keep on swimming.
The route he'll swim down the Thames is around 140 miles long. David started the challenge at the river's source in Gloucestershire and he will finish at Big Ben in London.
But he didn't last long in his swimming trunks - pretty soon he changed into a wetsuit because the Thames was very cold and his skin was turning blue.
Lots of people have lined the river bank to wish David luck with his challenge.
He also gets to munch on snacks to keep his energy levels up. He'll burn up 4,400 calories every day doing the swim.
David hopes to complete the big swim in eight days - and raise loads of money for charity. By Thursday, he'd already raised more than £300,000.