The Little Britain comic races up to join the five-strong male dance troupe in their routine to 'We're Sailor Men', a reworking of disco classic 'It's Raining Men'.
The group's sparkly hats attracted Walliams, who said: "I just want to join and sing that song with you."
After the performance, Walliams revealed, "I was in heaven", while fellow judge Amanda Holden claimed, "The Royal Family would adore you!"
However, an unsure Simon Cowell said: "Everything was rubbish but kind of good."
Britain's Got Talent airs Saturday at 8pm on ITV1.
Watch a clip of David Walliams and the Show Bears performing 'We're Sailor Men' below: