Radhanath Sikdar (Bengali: রাধানাথ শিকদার; 5 October 1813 – 17 May 1870) was an Indian mathematician and social reformer. He is best known for being the first person to calculate the height of Mount Everest, in 1852. He was a member of Henry Derozio's Young Bengal group. In 1854, along with fellow Derozian Peary Chand Mitra, Sikdar founded Masik Patrika, a Bengali journal for the education of the Indian women.

Sikdar was born on 5 October 1813, into a Bengali Brahmin family in Calcutta to Tituram Sikdar and Deviki Sikdar. His father worked as a clerk. He had a brother named Srinath Sikdar. Demonstrating exceptional mathematical ability from an early age, Radhanath's talent soon drew the attention of John Tytler, then Professor of Mathematics at Hindu College (now Presidency University). Recognizing his promise, Tytler provided him with invaluable mentorship and academic support, which played a formative role in Sikdar's intellectual development.

In 1831, George Everest, the Surveyor General of India, was in the pursuit of a mathematician who had specialised in spherical trigonometry, so that they could be a part of the Great Trigonometric Survey. In 1832, under the leadership of Everest, the longitudinal series of the "triangle" survey was completed from Sironj in Central India to Calcutta in Bengal.

While still working on mapping Calcutta, Bengal, Everest had begun his search for a mathematician, and soon enough, John Tytler, a professor of Mathematics at the Hindu College, now known as the Presidency College, recommended his 18-year-old pupil, Radhanath Sikdar.

Radhanath, a student of the college since 1824, was one of the first two Indians to read Isaac Newton's Principia and by 1832; he had studied Euclid's Elements, Thomas Jephson's Fluxion and Analytical Geometry and Astronomy by Windhouse. Taking inspiration from these prestigious papers, he devised a new method to draw a common tangent to two circles, when he was just a teenager. There was little doubt about Radhanath's proficiency in his subject, and he secured the job at the GTS on 19 December 1831 as a "computer" at a salary of thirty rupees per month.

Soon he was sent to Sironj near Dehradun. Even as seven other Bengali 'computers' worked alongside him, Radhanath soon showed his superior skills in mathematics and became Everest's favourite colleague. So much so, that he once stopped his transfer to another department. Radhanath's job was to carry geodetic surveys—the study of the earth's geometric shape orientation in space and gravitational field. He did not just use the established methods but invented his own to accurately measure these factors.

George Everest retired in 1843 and was succeeded by Colonel Andrew Scott Waugh. Eight years later, in 1851, Radhanath was promoted to the position of Chief Computer and transferred to Calcutta. Here, he was also a superintendent for the Meteorological Department.

At the order of Colonel Waugh, Radhanath started measuring the height of mountains. The brilliant mathematician, who had perhaps never seen Mount Everest, discovered in 1852 that Kangchenjunga, which was considered to be the tallest in the world, wasn't really so. Compiling data about Mount Everest from six observations, he eventually came to the conclusion that it was the tallest in the world.