Sarnaism is a religious faith of the Indian subcontinent, predominantly followed by indigenous communities of the Chota Nagpur Plateau region across states such as Jharkhand, Odisha, West Bengal, Bihar, and Chhattisgarh.

The essence of the Sarna faith revolves around nature worship. Its core principles emphasize jal (water), jaṅgal (forest) and jamīn (land), with adherents offering prayers to trees and hills while believing in the protection of forests. This belief centers around the reverence of Sarna, the sacred groves of village communities, where the village deity, known as Gram Deota, resides, and where sacrificial offerings are made twice a year. It is also referred to as "Sarna Dharma" or the "Religion of the Holy Woods", and it holds the distinction of being India's largest tribal religion.

The term "Sarna", derived from the Mundari language, means "sacred grove" and refers to the sacred grove where tribal religious practices take place. It is etymologically related to the name of the Sal tree. However, the term for a sacred grove varies among tribes; for instance, the Santals and Bhumijs call it Jaherthan, the Oraons call it Kurukh Kuti, and the Hos call it Desauli.

The socio-political movements that emerged among the tribal communities of the Chota Nagpur region in the late nineteenth century, such as the Birsa Munda movement, the Tana Bhagat movement, and the Dupub movement, shaped a collective tribal consciousness. Though their primary focus was on autonomy in response to colonial expansion, these movements also addressed the preservation of ancestral practices and belief systems by resisting external religious influences. This period laid the foundation for ethno religious identity among tribals, with emerging expressions such as Marangburu practices and Kherwarism among the Santal, Dupub faith among the Ho, early articulation of Sarna and Birsaite path among the Mundas.

Sarnaism, as a religious identity, emerged in the 1930s, following the assertion of a collective indigenous identity as Adivasi by tribal activists in the Chota Nagpur region of erstwhile Bihar. They articulated a unifying religious identity for the diverse, nature based ancestral practices of tribal communities, with the purpose of asserting Adivasi distinctiveness from mainstream religions of India such as Hinduism, Christianity, or Islam. However, initially, the unifying concept was prevalent among the Oraons of erstwhile Ranchi under the ethno-religious term Adivasi. Later, the Oraon and Ho communities accepted the Mundari ethno-religious term Sarna to represent their religious identity. It gained broader acceptance through socio-political development associated with leaders such as Jaipal Singh Munda, Raghunath Murmu, and Kartik Oraon. After India’s independence, it experienced steady growth, incorporating diverse tribal communities into its fold, including larger tribes like the Santal, who had initially refrained from accepting its tenets but gradually aligned with it over the following decades. Politically, Sarnaism grew alongside the Jharkhand movement, formulated by the Adivasi Mahasabha and its offspring political organization, the Jharkhand Party, which was centered on the motifs of tribal identity and the demand for autonomy. The movement culminated on 15 November 2000, the birth anniversary of Birsa Munda, when the tribal-concentrated southern region of Bihar was bifurcated to form the separate state of Jharkhand.

The adherents of Sarnaism believe in, worship, and revere a village deity as protector of village called Gaon khunt, Gram deoti, Marang Buru, Singbonga, or by other names by different tribes. Adherents also believe in, worship, and revere Dharti ayo or Chalapachho Devi, the mother goddess identified as the earth or nature.

Sarna is place of worship which is sacred grove in Chotanagpur. It is called Jaher than or Jaher gar among Santal, and can be found in villages. Sal trees are in the sacred grove. The ceremonies are performed by the whole village community at a public gathering with the active participation of village priests, Pahan and assistant Pujar in Chotanagpur. The priest is called Naike among Santal. The sthal typically has multiple trees like sal, mahua, neem, and banyan.

The main festival of Sarnaism is Sarhul, a festival in which devotees worship their ancestors. During the festival, the pahan brings three water pots to the sarna. If the water pots reduce in level, they believe the monsoon will fail, but if it stays the same the monsoon will come as normal. Men then offer sakua flowers and leaves.