The issue is affecting Llangorse Lake, in the Bannau Brycheiniog/Brecon Beacons National Park.
Natural Resources Wales (NRW) has advised members of the public to avoid contact with the algae.
“We’ve identified blue-green algae affecting Llangorse Lake near Talgarth in south Powys,” said NRW in a statement on its Facebook page on Friday.
“Warning signs have been installed.
“Please ensure you and your pets and livestock do not come into contact with the algae or the water, and make sure you maintain basic hand hygiene.”
NRW added: “Blue-green algae naturally occurs in inland waters, estuaries and the sea. Blooms can form when their numbers become excessive.
“Bloom and scum-forming blue-green algae can produce toxins. These toxins can be very dangerous to animals.
“In humans, they can cause rashes after skin contact and illnesses if swallowed.”
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