Lake Buyan is a significant freshwater lake located in the highland region of northern Bali, Indonesia. It is situated within the caldera of the ancient Bratan volcano, part of the Bedugul volcanic area. The lake lies at an approximate elevation of 1,200 meters above sea level, contributing to its notably cooler climate compared to the coastal regions of the island.

The lake is one of a trio of adjacent crater lakes, along with Lake Tamblingan and Lake Beratan (Bratan). These lakes are hydrologically connected and play a crucial role in the regional water system. Lake Buyan itself is the largest of the three, covering a surface area of approximately 3.9 square kilometers. It serves as a vital reservoir for irrigation and drinking water for the surrounding districts in Buleleng Regency.

The area surrounding Lake Buyan is characterized by dense tropical montane rainforest, which is protected as part of the Bali Barat National Park extension. This ecosystem supports a variety of flora and fauna, including several endemic bird species. The lake and its environs are recognized for their high biodiversity and ecological importance.

Historically, the region around the lake, along with Lake Tamblingan, is associated with the ancient Tamblingan civilization, which dates back to the 10th to 14th centuries. Archaeological remains, including temple sites and meditation caves, can be found in the surrounding forest, indicating the area's long-standing spiritual significance.

Lake Buyan is a designated ecotourism and conservation zone. Primary activities are nature-based, focusing on the preservation of the environment. The lake is not developed for mass water sports or recreational swimming. Access is provided for quiet observation, hiking along designated forest trails, and bird watching. A notable point of access and panoramic view is the nearby "Hidden Hill" or "Bukit Asah" area.

The management of Lake Buyan emphasizes environmental protection due to its critical function as a water catchment. Regulations are in place to minimize pollution and disturbance to the natural habitat. The lake's health is directly linked to the water security and agricultural sustainability of northern Bali.