Jyeṣṭhadeva (Malayalam: ജ്യേഷ്ഠദേവന്) (c. 1500 – c. 1575)^[1][2] was an astronomer-mathematician of the Kerala school of astronomy and mathematics founded by Madhava of Sangamagrama (c. 1350 – c. 1425). He is best known as the author of Yuktibhāṣā, a commentary in Malayalam of Tantrasamgraha by Nilakantha Somayaji (1444–1544). In Yuktibhāṣā, Jyeṣṭhadeva had given complete proofs and rationale of the statements in Tantrasamgraha. This was unusual for traditional Indian mathematicians of the time. An analysis of the mathematics content of Yuktibhāṣā has prompted some scholars to call it "the first textbook of calculus".^[3] Jyeṣṭhadeva also authored Drk-karana a treatise on astronomical observations.^[4]

There are a few references to Jyeṣṭhadeva scattered across several old manuscripts.^[1] From these manuscripts, one can deduce a few bare facts about the life of Jyeṣṭhadeva. He was a Nambudiri belonging to the Parangngottu family (Sanskrtised as Parakroda) born about the year 1500 CE. He was a pupil of Damodara and a younger contemporary of Nilakantha Somayaji. Achyuta Pisharati was a pupil of Jyeṣṭhadeva. In the concluding verse of his work titled Uparagakriyakrama, completed in 1592, Achyuta Pisharati has referred to Jyeṣṭhadeva as his aged benign teacher. From a few references in Drkkarana, a work believed to be of Jyeṣṭhadeva, one may conclude that Jyeṣṭhadeva lived up to about 1610 CE.

Parangngottu, the family house of Jyeṣṭhadeva, still exists in the vicinity of Trikkandiyur and Alathiyur.^[1] There are also several legends connected with members of Parangngottu family.

Little is known about the mathematical traditions in Kerala prior to Madhava of Sangamagrama. Vatasseri Paramesvara was a direct disciple of Madhava. Damodara was a son of Paramesvara. Nilakantha Somayaji and Jyeshthadeva were pupils of Damodara. Jyeṣṭhadeva's pupil was Achyuta Pisharati and Melpathur Narayana Bhattathiri was Achyuta Pisharati's student.

Jyeṣṭhadeva is known to have composed only two works, namely, Yuktibhāṣā and Drkkarana. The former is commentary with rationales of Tantrasamgraha of Nilakantha Somayaji and the latter is a treatise on astronomical computations.

Three factors make Yuktibhāṣā unique in the history of the development of mathematical thinking in the Indian subcontinent: