Morro do Diabo State Park, State park in Teodoro Sampaio, São Paulo, Brazil

Morro do Diabo State Park covers 33,845.33 hectares of seasonal semi-deciduous Atlantic Forest in Teodoro Sampaio, at the far west of São Paulo state. Its canopy loses part of its leaves in the dry season and shelters peroba-rosa, cedar, ipê and pau-marfim. The Morro do Diabo hill rises 600 metres above sea level within the forest.

The forest reserve was established in 1941 as a protection zone for the region's vegetation and wildlife. Its creation followed the early 20th century arrival of a railway that had begun to open up the area for settlement.

The name Morro do Diabo (Devil's Hill) refers to the 600-metre hill inside the park; its origin is unproven, with legends of an indigenous burial ground or of revenge killings of white men during the colonial period. The park supports environmental education and scientific research, and visitors come for the black lion tamarin, rediscovered here after being thought extinct.

The park lies in Teodoro Sampaio, 687 km from the city of São Paulo, in the Pontal do Paranapanema. Access is by road from Teodoro Sampaio; the park supports environmental education and scientific research. Rainfall concentrates in summer, averaging 1,100 to 1,300 mm, and daily temperatures run from about 13 °C in May–August to 32 °C in January–March.

The park is home to one of the largest populations of black lion tamarins, a threatened primate species found nowhere else in large numbers. These small monkeys hide in the forest canopy and are rarely seen by visitors despite living throughout the area.