The 'Tik Tok' singer, who was at the Shepherds' Tavern in Mayfair with Lily Allen, allegedly pulled down her wet-look leggings and shocked other punters as she relieved herself in front of them.
"When you've got to go, you've got to go, girls," The Mirror quotes her as saying.
Allen then responded: "Now even I couldn't do that. Oh my God."
The 22-year-old, who will support Calvin Harris and Mr Hudson on their upcoming live dates, reportedly offended other drinkers at the venue.
"That is the most disgusting thing I've ever seen. I can't believe she just peed in the sink. Where am I supposed to wash my hands now?" a source said.
Ke$ha is featured on Flo Rida's single 'Right Round', which topped the charts in the UK in March.