Sadhu (Sanskrit: साधु, IAST: sādhu (male), sādhvī or sādhvīne (female), also spelled saddhu) is a religious ascetic, mendicant or any holy person in Hinduism and Jainism who has renounced the worldly life. They are sometimes alternatively referred to as yogi, sannyasi or vairagi.

Sādhu means one who follows a path of sadhana(spiritual discipline). Although the vast majority of sādhus are yogīs, not all yogīs are sādhus. A sādhu's life is solely dedicated to achieving mokṣa (liberation from the cycle of death and rebirth), the fourth and final aśrama (stage of life), through meditation and contemplation of Brahman. Sādhus often wear simple clothing, such as saffron-coloured clothing in Hinduism and white or nothing in Jainism, symbolising their sannyāsa (renunciation of worldly possessions). A female mendicant in Hinduism and Jainism is often called a sadhvi, or in some texts as aryika.

In Sikhism, a person who has become Brahmgiani is considered a sadhu. However, asceticism, celibacy and begging are prohibited in Sikhism.

The term sadhu (Sanskrit: साधु) appears in Rigveda and Atharvaveda where it means "straight, right, leading straight to goal", according to Monier Monier-Williams. In the Brahmanas layer of Vedic literature, the term connotes someone who is "well disposed, kind, willing, effective or efficient, peaceful, secure, good, virtuous, honourable, righteous, noble" depending on the context. In the Hindu Epics, the term implies someone who is a "saint, sage, seer, holy man, virtuous, chaste, honest or right".

The Sanskrit terms sādhu ("good man") and sādhvī ("good woman") refer to renouncers who have chosen to live lives apart from or on the edges of society to focus on their own spiritual practices.

The words come from the root sādh, which means to "reach one's goal", "straighten", or "master". The same root is used in the word sādhanā, which means "spiritual practice".

Sadhus are widely considered holy. It is also thought that the austere practices of the sadhus help to burn off their karma and that of the community at large. Thus seen as benefiting society, sadhus are supported by donations from many people. However, reverence of sadhus is by no means universal in India. For example, Nath yogi sadhus have been viewed with a certain degree of suspicion particularly amongst the urban populations of India, but they have been revered and are popular in rural India.

There are naked (digambara, or "sky-clad") sadhus who wear their hair in thick dreadlocks called jata. Sadhus engage in a wide variety of religious practices. Some practice asceticism and solitary meditation, while others prefer group praying, chanting or meditating. They typically live a simple lifestyle, and have very few or no possessions. Many sadhus have rules for alms collection, and do not visit the same place twice on different days to avoid bothering the residents. They generally walk or travel over distant places, homeless, visiting temples and pilgrimage centers as a part of their spiritual practice. Celibacy is common, but some sects experiment with consensual tantric sex as a part of their practice. Sex is viewed by them as a transcendence from a personal, intimate act to something impersonal and ascetic.