Lake Rotomahana is an 800-hectare lake in northern New Zealand, located 20 kilometres to the south-east of Rotorua. It is immediately south-west of the dormant volcano Mount Tarawera, and its geography was substantially altered by a major 1886 eruption of Mount Tarawera. Along with the mountain, it lies within the Okataina caldera.
The New Zealand Ministry for Culture and Heritage gives a translation of "warm lake" for Rotomahana, following Hochstetter.
Before the 1886 eruption, only two small lakes were present in the current lake's basin. Following the eruption, a number of craters filled over the course of 15 years to form today's Lake Rotomahana. It is the most recently formed larger natural lake in New Zealand, and the deepest in the Rotorua district. The lake's northern shore lies close to the 39 metres lower Lake Tarawera, separated by less than 700 metres of terrain that is mostly material from the 1886 eruption. Lake Rotomahana has no natural outlet, and its water level varies by about one meter in response to rainfall and evaporation.
The lake is a wildlife refuge, with all hunting of birds prohibited.