Leeds City Council's parks department said around 15 life rings and 15 life lines have been taken from Waterloo Lake at Roundhay Park over the past month.
A spokesman said: "The life -saving equipment is essential and should not be removed for any purpose other than assisting persons in difficulty in the water.
"Removal of this could cause a potential drowning if no equipment is available.
"We do life line checks on a daily basis and accept some problems, but it seems to have escalated over the past month resulting."
The equipment stolen will cost around Â£600 to replace, with the majority of items having been stolen during the last two weeks.
Police are now appealing for information on the thefts and urging the public to report anyone messing around with the equipment.
Call the Leeds Inner North East team via 101, quoting 13170106532.