Udasis (Gurmukhi: ਉਦਾਸੀ ਸੰਪਰਦਾ; udāsī saparadā) (Devanagari: उदासी संप्रदाय), also spelt as Udasins, also known as Nanak Putras (meaning "sons of Nanak"), are a religious sect of ascetic sadhus centred in northern India who follow a tradition known as Udasipanth. Becoming custodians of Sikh shrines in the 18th century, they were notable interpreters and spreaders of the Sikh philosophy during that time. However, their religious practices border on a syncretism of Sikhism and Hinduism, and they did not conform to the Khalsa standards as ordained by Guru Gobind Singh. When the Lahore Singh Sabha reformers, dominated by Tat Khalsa Sikhs, would hold them responsible for indulging in ritual practices antithetical to Sikhism, as well as personal vices and corruption, the Udasi mahants were expelled from the Sikh shrines.

Udasi and Udasin is derived from the Sanskrit word Udāsīn, which means one who is indifferent to or disregardful of worldly attachments, a stoic, or a mendicant. The word Udasi is derived from the Sanskrit word udasin, meaning 'detached, journey', reflecting an approach to spiritual and temporal life, or from udas ('detachment'), signifying indifference to or renunciation of worldly concerns.

According to myth, the sect was established in the Puranic age but historically speaking, the sect was founded by and based on the teachings of Guru Nanak's elder son Sri Chand (1494–1629, other sources give a death year of 1643). Sri Chand, contrary to his father's emphasis on participation in society, propagated ascetic renunciation and celibacy. Another Sikh tradition links the Udasis to Baba Gurditta, the eldest son of Guru Hargobind, and there is dispute on whether the Udasis originated with Sri Chand or Gurditta. Another viewpoint is that Sri Chand was the founder of the sect and passed the leadership to Baba Gurditta as his successor.

They maintain their own parallel line of gurus from Guru Nanak, followed by Sri Chand, followed by Gurditta. They first came to prominence in the 17th century, and gradually began to manage Sikh shrines and establishments in the 18th century, from where they espoused a model of Sikhism that diverged considerably from that of the Khalsa. The Udasis started receiving state-patronage from Sikh rulers in the late 18th century, with revenue-free land being bestowed to them. This allowed the Udasis to establish their akharas, many of which were founded around the same time, such as the Brahm Buta near Darbar Sahib in Amritsar.

They would set up establishments across North India through to Benares, where they would come to be joined with monastic asceticism. The combination of Hindu gods and the Sikh religious text indicated that the sect evolved over time under many historical influences and conditions, interpreting the message of Guru Granth Sahib in monistic Vedantic terms. They were initially largely based in urban centers where they set up their establishments, or akharas, only beginning to spread into rural areas during Sikh rule; before, they had around a dozen centres; by the end of Maharaja Ranjit Singh's reign, the number had increased to around 250. The Udasis widely propagated its form of Sikh philosophy, and during the 18th and early-19th centuries, their teachings attracted a large number of people to the Sikh fold. According to early Gurbilas literature and some modern scholars, Guru Gobind Singh had employed a large number of armed, militant Udasi asectics prior to the construction of the forts of Anandpur Sahib. It has been posited that Guru Gobind Singh initiated the Khalsa in order to amalgamate the nirgun bhakti beliefs of the Ramanandis and the martial traditions of the growing number of armed mahants.

Before the emergence of the Singh Sabha Movement in the late 19th century, they controlled important Sikh shrines, including the Harimandir Sahib, generally known also as the Golden Temple, for a short while. However, during the Akali movement of the 20th century, the Tat Khalsa Sikhs expelled them from the Sikh shrines, accusing them of vices and of indulging in ritual practices that were against the teachings of the Sikh gurus. The Sikh Gurdwara Reform Act, 1925 defined the term "Sikh" in a way that excluded the syncretic groups like Udasis, Nanakpanthis, and other groups who maintained transitional identities. Subsequently, the Udasis increasingly identified themselves as Hindus rather than Sikhs.

According to 18th-century descriptions, they either cut or matted their hair under a turban, rather than knot it under a turban like Khalsas, and instead of the Khalsa emphasis on the panj kakkar garb and sporting arms, their dress code would include items such as a cap, a cotton bag, a flower rosary, a vessel made of dried pumpkin, a chain around the waist, ash to smear on their body, and a deerskin upon which to perform hatha yoga, resulting in an extremely divergent appearance from Khalsa Sikhs in the eighteenth century. In addition to not consider the Khalsa's Rehat Maryada to be binding on them, their modes of thought and attitude towards salvation also differed significantly. The Khalsa believed that salvation could be attained while taking part in society and pursuing secular objectives like political power and accumulation of resources like agrarian land, though this had to be accomplished within a particular framework of beliefs and spiritual practices, chief among which was the societal order and structure of the Khalsa. The Udasis considered secular pursuits to be incompatible with personal salvation, which was to be achieved only through renouncing the world, espousing asceticism and a monastic traveler lifestyle. Udasis are known for their akharas along with the Nirmala sect of Sikhism.

The Udasis also worship the panchayatana, the five Hindu deities: Shiva, Vishnu, Durga, Ganesha, and Surya.