Blue Lake or Rotomairewhenua is a small lake in Nelson Lakes National Park, in the northern reaches of New Zealand's Southern Alps. Sacred to local Māori, it has the clearest natural fresh water in the world.
Blue Lake is most often reached as a side trip from the Travers-Sabine Circuit, although a marked route to the south, over the Waiau Pass and another to the West, via Moss Pass continues beyond the lake. The nearby Blue Lake Hut, with 16 bunks, provides accommodation for more than 700 trampers each year.
The Blue Lake is drained by the west branch of the Sabine River, which is part of the Buller River system. It is fed by a short upper segment of the Sabine, which in turn is fed by underground seepage through the landslide debris impounding the much larger Lake Constance. Blue Lake is roughly boomerang shaped, running north then northwest, with each arm of the lake stretching some 200 metres (700 ft).
The lake has extremely clear water, and is the clearest natural body of fresh water yet reported. A 2011 study found its visibility ranged from 70 to 80 metres (230 to 260 ft), clearer than the 63 metres (207 ft) measured for Te Waikoropupu Springs, a previous record holder.