David Walliams said completing his 140-mile charity swim along the River Thames would be "meaningless" without supporters who donated more than £1 million.
Thousands of fans gathered at Westminster Bridge to cheer as the funnyman finished his gruelling eight-day journey on Monday.
Afterwards, he joked: "I think a bath is the only water I want to see for quite a while."
Among those who welcomed him were his wife Lara Stone, comedian and actor Lenny Henry, actress Barbara Windsor, comic Miranda Hart and Olympic rower Steve Redgrave.
When asked by Henry what he would like to say to his supporters, he said: "Thank you so much, thank you for your waves, your cheers, but most of all thank you for your donations, because each time this would be completely meaningless unless you all put your hands in your pocket, and I know times are hard so I really, really thank you from the heart, you're really going to make people's lives better."
Walliams, 40, set off from Lechlade in Gloucestershire on Monday and had to contend with not only the aches and pains of the swim, but also diarrhoea, vomiting and low energy levels from "Thames tummy". He also struggled with a painful rash where his wetsuit rubbed his neck after the cold water forced him to abandon plans to swim in trunks.
Organisers of the BT Sport Relief Challenge said Walliams's swim had raised £1,093,325 by the time he finished, with more continuing to come in.
He burned nearly 66,000 calories as he powered through 107,062 strokes on the 140.54-mile trip.
"It wasn't really fun, it was 140 miles swimming in cold water so the fun bit is when you get out," he said. "You are alone with your thoughts for a really long time. Some days I was in the water for 10-12 hours so you sometimes have delusional thoughts so you try to think of songs that you love or movies that you love."
In 2006 Walliams swam the Channel, raising £1 million in aid of Sport Relief. He has also swum the Straits of Gibraltar and last year cycled from John O'Groats to Land's End, also for Sport Relief.