Vaishnavism (Sanskrit: वैष्णवसम्प्रदायः, romanized: Vaiṣṇavasampradāyaḥ), also called Vishnuism, is one of the major Hindu traditions, that considers Vishnu as the sole supreme being leading all other Hindu deities, that is, Mahavishnu. It is one of the major Hindu denominations along with Shaivism, Shaktism, and Smartism. Its followers are called Vaishnavites or Vaishnavas (IAST: Vaiṣṇava), and it includes sub-sects like Krishnaism and Ramaism, which consider Krishna and Rama as the supreme beings respectively. According to a 2020 estimate by The World Religion Database (WRD), hosted at Boston University's Institute on Culture, Religion and World Affairs (CURA), Vaishnavism is the largest Hindu sect, constituting about 399 million Hindus.

The ancient emergence of Vaishnavism is unclear, and broadly hypothesized as a fusion of various regional non-Vedic religions with worship of Vishnu. It is considered a merger of several popular non-Vedic theistic traditions, particularly the Bhagavata cults of Vāsudeva-Krishna and Gopala-Krishna, as well as Narayana, developed in the 7th to 4th century BCE. It was integrated with the Vedic God Vishnu in the early centuries CE, and finalized as Vaishnavism, when it developed the avatar doctrine, wherein the various non-Vedic deities are revered as distinct incarnations of the supreme God Vishnu. Narayana, Hari, Rama, Krishna, Kalki, Perumal, Shrinathji, Vithoba, Venkateswara, Guruvayurappan, Ranganatha, Jagannath, Badrinath and Muktinath are revered as forms or avatars of Vishnu, all seen as different aspects of the same supreme being.

The Vaishnavite tradition is known for the loving devotion to an avatar of Vishnu (often Krishna), and as such was key to the spread of the Bhakti movement in the Indian subcontinent in the 2nd millennium CE. It has four Vedanta—schools of numerous denominations (sampradaya): the medieval-era Vishishtadvaita school of Ramanuja, the Dvaita school of Madhvacharya, the Dvaitadvaita school of Nimbarkacharya, and the Shuddhadvaita of Vallabhacharya. There are also several other Vishnu-traditions. Ramananda (14th century) created a Rama-oriented movement, now the largest monastic group in Asia.

Key texts in Vaishnavism include the Vedas, the Upanishads, the Bhagavad Gita, the Pancharatra (Agama) texts, Naalayira Divya Prabhandham, and the Bhagavata Purana.

The ancient emergence of Vaishnavism is unclear, with evidence inconsistent and scanty. Syncretism of various traditions resulted in Vaishnavism. Although Vishnu was a Vedic solar deity, he is mentioned less often than Agni, Indra, and other Vedic deities, thereby suggesting that he had a minor position in the Vedic religion.

According to Dandekar, what is understood today as Vaishnavism did not originate in Vedism, but emerged from the merger of several popular theistic traditions after the decline of Brahmanism at the end of the Vedic period, shortly before the second urbanisation of northern India, in the 7th to 4th century BCE. It initially formed as Vasudevism around Vāsudeva, a deified leader of the Vrishnis. Later, Vāsudeva was amalgamated with Krishna "the deified tribal hero and religious leader of the Yadavas", to form the merged deity Bhagavan Vāsudeva-Krishna, due to the close relation between the tribes of the Vrishnis and the Yadavas. This was followed by a merger with the cult of Gopala-Krishna of the cowherd community of the Abhıras in the 4th century CE. The character of Gopala Krishna is often considered to be non-Vedic. According to Dandekar, such mergers positioned Krishnaism between the heterodox sramana movement and the orthodox Vedic religion. The "Greater Krsnaism", states Dandekar, then adopted the Rigvedic Vishnu as Supreme deity to appeal to orthodox elements.

According to Klostermaier, Vaishnavism originates in the last centuries BCE and the early centuries CE, with the cult of the heroic Vāsudeva, later amalgamated with Krishna of the Yadavas, and still several centuries later with the "divine child" Bala Krishna of the Gopala traditions. According to Klostermaier, "In some books Krishna is presented as the founder and first teacher of the Bhagavata religion." According to Dalal, "The term Bhagavata seems to have developed from the concept of the Vedic deity Bhaga, and initially it seems to have been a monotheistic sect, independent of the Brahmanical pantheon."

This complex history is reflected in the two main early denominations of Vishnavism: the Bhagavats, worshipping Vāsudeva-Krishna and follow Brahmanic Vaishnavism, and the Pacaratrins, who regarded Narayana as their founder and follow the Tantric Vaishnavism.