Thomas Martin Lowry (October 26, 1874 – November 2, 1936) was an English physical chemist.

He was born in Low Moor, Bradford, West Yorkshire, England.

He studied chemistry under Henry Armstrong, an English chemist whose interests were primarily in organic chemistry but also included the nature of ions in aqueous solutions.

In 1896 he became Armstrong's assistant. In 1898, he noted the change in optical rotation on nitro-d-camphor with time and invented the term mutarotation to describe this phenomenon. In 1906 became a lecturer in chemistry at Westminster Training College. In 1912 he moved to Guy's Hospital Medical School, becoming the head of the chemistry department in 1913 and the first professor of chemistry in any London medical school.

In 1914 he was elected Fellow of the Royal Society. In 1920 he became the first holder of a chair of physical chemistry at Cambridge University.

He studied changes in optical rotation caused by acid- and base-catalyzed reactions of camphor derivatives, which led to his formulation of the protonic definition of acids and bases in 1923, independently of the advocacy of the same concept by Johannes Nicolaus Brønsted in the same year.

He continued at Cambridge for the rest of his life.