The Sierra de San Juan Biosphere Reserve is a protected area in Nayarit state of western Mexico.

It protects a portion of the Sierra de San Juan, a mountain range which lies between Tepic and the Pacific Ocean.

The reserve protects the Sierra de San Juan, a western extension if the Trans-Mexican Volcanic Belt.

[2] The eastern slopes drain into the valley of the Mololoa River, a northward-flowing tributary of the Río Grande de Santiago.

Volcán San Juan has an oval-shaped caldera, one by four kilometers, which formed in an eruption about 15,000 years ago.

Native birds include Mexican woodnymph (Thalurania ridgwayi), crested guan (Penelope purpurascens), prairie falcon (Falco mexicanus), common black hawk (Buteogallus anthracinus), hooded oriole (Icterus cucullatus), bumblebee hummingbird (Selasphorus heloisa).

The area was decreed a reserva de conservación y equilibrio ecológico y regeneración del medio ambiente (reserve for conservation, ecological balance, and environmental regeneration).

On the drier northern and eastern slopes the lower boundary was raised to the 1080-meter contour, which left some urbanized and cultivated areas outside the reserve.