Kalpathi Ramakrishna Ramanathan (28 February 1893 – 31 December 1984) was an Indian physicist and meteorologist. He was the first Director of Physical Research Laboratory, Ahmedabad. Ramanathan was awarded Padma Bhushan in 1965 and Padma Vibhushan in 1976.
Born in Kalpathi, Palakkad, to a Sanskrit scholar, Ramanathan received a B.A. degree from the Government Victoria College, Palakkad and an M.A. degree in physics from the Presidency College, Madras. He began his academic career as a demonstrator in physics at the Maharajah’s College of Science in Thiruvananthapuram. He was later appointed as the Honorary Director of Observatory in Thiruvananthapuram.
In 1921, Ramanathan moved to Calcutta to collaborate with C. V. Raman on the studies of X-ray diffraction in the liquids. For this work he received D.Sc. degree from University of Madras. This was the first time that a D.Sc. degree was awarded by University of Madras.
In 1922 Ramanathan joined Rangoon University as Assistant Professor of physics. In 1925 he was appointed as a senior scientist in the India Meteorological Department and remained there until his retirement in 1948. The same year he joined the Physical Research Laboratory (PRL), Ahmadabad as its first Director. He established Dobson ozone spectrophotometer stations in India. He retired in 1966, but continued to serve PRL as Emeritus Professor until his death in 1984 at the age of 91.