Tulasi (Sanskrit: तुलसी, romanized: Tulasī), Tulsi or Vrinda (holy basil) is a sacred plant in Hindu tradition. Hindus regard it as an earthly manifestation of the goddess Tulasi; she is regarded as the avatar of Lakshmi, and thus the consort of the god Vishnu. In another iteration, as Vrinda, she is married to Jalandhara. The offering of its leaves is recommended in ritualistic worship of Vishnu and his avatars, like Krishna and Vithoba.

Traditionally, the tulasi is planted in the center of the central courtyard of Hindu houses. The plant is cultivated for religious purposes, and for its essential oil.

In the Vedas, Tulasi ("matchless") is known as Vaishnavi ("belonging to Vishnu"), Vishnu Vallabha ("beloved of Vishnu"), Haripriya ("beloved of Vishnu"), Vishnu Tulasi. The Tulasi with green leaves is called Shri-Tulasi ("fortunate Tulasi") or Lakshmi-Tulasi; Shri is also a synonym for Lakshmi, Vishnu's spouse. This variety is also known as Rama-Tulasi ("bright Tulasi"); Rama is also one of the principal avatars of Vishnu. The Tulasi with dark green or purple leaves and purple stem is called Shyama-Tulasi ("dark Tulasi") or Krishna-Tulasi ("dark Tulasi"); Krishna is also a prominent avatar of Vishnu. This variety is considered especially sacred to Krishna, as its purple color is similar to Krishna's dark complexion.

One argument mooted is that goddess Lakshmi is also identical with Tulasi and hence it is also known as Lakshmi Priya ("beloved Lakshmi"). Tulasi is also identified with the wives of other incarnations of Vishnu, such as of Rama and Krishna.

The name Tulasi is also believed to literally translate as "immeasurable". Tula means a scale or balance, where an item is place on one side and weights on the other to compare and measure the weight. Hence, Tulasi could also mean the one who cannot be measured or compared.

The Devi Bhagavata Purana regards Tulasi as a manifestation of Lakshmi, the goddess of wealth and prosperity, and the principal consort of Vishnu. Once upon a time, King Vrishadhvaja—a devotee of the god Shiva—banned worship of all other deities except for that of his patron god. The agitated sun god, Surya, cursed him that he would be abandoned by Lakshmi. Upset, Shiva pursued Surya, who fled, finally seeking shelter with Vishnu. Vishnu said to the deities that years had passed on earth. Vrishadhvaja and also his heir-son were dead and his grandchildren—Dharmadhvaja and Kushadhvaja—were now worshiping Lakshmi to gain her favor. Lakshmi rewarded their efforts by being born as their daughters Tulasi to Dharmadhvaja, and Vedavati to Kushadhvaja, respectively. In time, Tulasi gave up all her royal comforts, and went to Badrinath to perform a penance to gain Vishnu as her husband. The god Brahma was pleased with her penance, but told her that she would have to marry the daitya Shankhacuda (The reincarnation of Sridama, a devotee of Krishna) before she could marry Vishnu. Shankhacuda is regarded as an Aṃśāvatāra of Vishnu.

Shankhacuda, a mighty daitya, underwent a severe tapasya that pleased Brahma. He was granted the Viṣṇukavaca (the armour of Vishnu), and was blessed with another boon: As long as the Viṣṇukavaca was on his body, no one could slay him. Shankhacuda and Tulasi were soon married. His arrogance caused conflict with the devas, who petitioned Vishnu to relieve them. Vishnu granted his trishula to Shiva, who slew Shankachuda with the weapon. Vishnu appeared in the form of Shankacuda to Tulasi, and the two sported, and ceased when Tulasi realised that he was an imposter. When she jumped to curse him, Vishnu appeared as his true form and spoke to her, after which Tulasi joined him to depart to Vaikuntha:

You have been doing penance for a long time to get me as your husband. Your husband Śaṅkhacūḍa was the chief of my Pārṣadas, Sudāmā. It is time for him to go back to Goloka getting himself released from the curse. By this time Śiva would have killed him and he would have gone to Goloka as Sudāmā. You can now abandon your body and come with me to Vaikuṇṭha to enjoy life as my wife. Your body will decay and become a holy river named Gaṇḍakī; your hair will become the Tulasī plant, the leaves of which will be held sacred in all the three worlds.