It was discovered by Leeds City Council's director of sport John Hammond when he was rummaging through the storeroom at the baths on Holbeck Lane shortly after it closed in 1979.
Mr Hammond said: "I think signs like this must have been put up in the 1920s and 30s. I can only assume that people arrived to swim in a grubby state, hence the need for a notice. But I do not understand the reason for the instruction on washing kneecaps and elbows.
"I can think of other parts of the body that are more vital to keep clean, but they are not listed here. In those days I think people went for a swim to get clean, because most would have no baths in their homes and this must have caused problems for the people who looked after the pools."
He added: "Today it seems comical that people needed to be told how to wash themselves."
The notice from the former Holbeck Baths was due to go on display in the industrial and science museum at Armley Mills.