Police were called to Ardsley Reservoir in Tingley on the evening of Friday, May 29.
A group of people were swimming in the water and one of them required rescuing.
Emergency services were called to the scene.
The swimmer was taken to hospital due to low blood pressure and inhaling a lot of water.
Officers from the Leeds south neighbourhood team said they were "lucky" to have survived and are "doing ok".
Police have now issued a stark warning to people who are considering swimming in the reservoir.
A WYP Leeds South spokesman said: "Ardsley Reservoir, Tingley. !!NO SWIMMING!!
"We understand how tempting swimming in water can be, especially whilst we are having some hot weather.
"But we are telling you, not to!
"Last night there was an incident at Ardsley Reservoir which involved a number of people swimming in the water.
"This resulted in someone needing rescuing from the water, mbulance attention and assistance from a number of officers and emergency services.
"They were taken to hospital due to low blood pressure and they had inhaled a lot of water.
"Luckily, on this occasion they survived and are doing ok.
"That is not always the case, people have died from swimming in reservoirs.
"We cannot stress to you enough how dangerous it is!
"It may look beautiful but it can be deadly!
"Cold water shock can kill you in 60 seconds!
"!!DO NOT SWIM IN THE RESERVIOR!!"
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