Lake Llew was a land-locked body of water in an unknown region of Prydain.
According to the three enchantresses, the lake was once inhabited by a salmon that might have been enchanted, because it was wise enough to answer supplicants' questions. Orwen said of it: "I've never met a wiser fish." However, Orgoch explained that the salmon was "long gone".
Llyn Llyw (English: Lake Llyw) appears in the tale "Culhwch and Olwen" in the collection of Welsh legends called the Mabinogion. During a quest the heroes are led by a salmon to the prison of Mabon, deep underwater in the lake.