The Little Britain comedian was met by a Labrador named Vinny as he continued his eight-day 140-mile journey, but was forced to step in and help the pooch when it struggled to swim back to the river bank.
"We had a little incident with a dog on the way to Maidenhead," Walliams wrote on Twitter. "I heard a splash and looked over to be met with the face of a sweet Labrador.
"I thought he was fine, but he started to really struggle when he was getting out so I swam over and helped his owner get him out."
Watch Miranda Hart cook David Walliams breakfast on Day 6 of his swim below:
Vinny's owner Steve Outten today told told the Daily Mail that he was "amazed" by Walliams's heroic act.
"Vinny has hip dysplasia and was really struggling to get out just as David was swimming past," he said.
"He stopped, swam over and helped me get him out - which when you think what he's going through with the swim is totally amazing. What a man."
Walliams was crippled by severe vomiting and diarrhoea after contracting a stomach bug during his swim this week.
Watch yesterday's highlights of David Walliams's swim below: