David Walliams said he felt relieved after completing a 140-mile swim along the length of the River Thames.
He reached Westminster Bridge eight days after starting near the river's source, during which he battled illness and rescued a dog.
By the time he finished Walliams had raised more than £1m for the Big Splash Challenge for Sport Relief.
After his swim, Walliams told the BBC: "A bath is the only water I want to see for quite a while."
The comedian had already completed swims across the English Channel in 2006 and the Strait of Gibraltar in 2008 in previous charity events.
The Little Britain star entered the water at Teddington Lock on Monday morning, completing a further 15 miles to reach his goal.
The last leg of his challenge saw him tackle the tidal stretch of the Thames, with strong currents and undertows, along with busy river traffic.
Walliams was greeted on Westminster Bridge by his wife Lara Stone who has been following his progress in a support boat.
Fellow comedians Lenny Henry and Miranda Hart and Olympic rower Steve Redgrave were among the huge crowd cheering him on.
As he clambered out of the water just before 18:30 BST towards a podium at County Hall on the southern bank of the Thames, there was a shower of red, white and blue ticker tape.
He said as he crossed the finish, there was a wave of "relief that I didn't have to do any more".
He began near Lechlade, in Gloucestershire, on 5 September but soon suffered a bout of "Thames tummy" which left him with a high temperature.
"It was hard work," Walliams told BBC News.
"It was really, really tough and the distance took it out of me. I did a lot of training but I don't know quite how you can prepare for this because it's an endurance event."
He said it was "beautiful" coming into London and seeing the Houses of Parliament in the sun.
"I felt really proud to be British and really proud that all the people cared," he said.
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.