Sannyasa (Sanskrit: संन्यास; IAST: Saṃnyāsa), sometimes spelled Sanyasa or Sanyasi (for the person), is life of renunciation and the fourth stage within the Hindu system of four life stages known as Ashramas, with the first three being Brahmacharya (bachelor student), Grihastha (householder) and Vanaprastha (forest dweller, retired). Sannyasa is traditionally conceptualized for men or women in late years of their life, but young brahmacharis have had the choice to skip the householder and retirement stages, renounce worldly and materialistic pursuits and dedicate their lives to spiritual pursuits.

Sannyasa, a form of asceticism marked by renunciation of material desires and prejudices, is represented by a state of disinterest in and detachment from material life, whose purpose is spending one's life in peaceful, spiritual pursuits. An individual in Sanyasa is known as a sannyasi (male) or sannyasini (female) in Hinduism. Sannyasa shares similarities with the Sadhu and Sadhvi traditions of Jain monasticism, and the sannyasi and sannyasini share similarity with the bhikkhus and bhikkhunis of Buddhism.

Sannyasa has historically been a stage of renunciation, ahimsa (non-violence) peaceful and simple life and spiritual pursuit in Indian traditions. However, this has not always been the case. After the invasions and establishment of Muslim rule in India, from the 12th century through the British Raj, parts of the Shaiva (Gossain) and Vaishnava (Bairagi) ascetics metamorphosed into a military order, where they developed martial arts, created military strategies, and engaged in guerrilla warfare. These warrior sanyasi (ascetics) played an important role in helping European colonial powers establish themselves in the Indian subcontinent.