Posters have been put up in Burgess Park advising people to keep themselves and their pets from coming into contact with the water due to naturally occurring algae.
Southwark Council said the algae is being monitored for bacteria levels.
Current readings show the risk of coming into contact with harmful bacteria is low.
But as a precaution the council is encouraging people to keep a wide berth – including their pets.
Cllr Rebecca Lury, Cabinet Member for culture, leisure, equalities and communities, said: “Low level, naturally occurring algae blooms have been identified on the lake.
“These are extremely common in the UK this year and we are currently working with the Environment Agency and monitoring the water.
“Initial analysis of our water sample indicates that the risk of potentially harmful bacteria is low.
“But, as a precaution, we have put signs up around the lake, asking people to protect themselves and their animals, by keeping away from the algae and the water.”