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

Sādhu means one who practises a 'sadhana' or keenly follows a path of 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.