Theodore William Richards was an American chemist.

He was born in Germantown, Pennsylvania on 31 January 1868. His parents were William T. Richards, a land- and seascape painter, and Anna née Matlack, a poet.

He was educated at first by his mother, and travelled to England and France. In 1883 he entered Haverford College, Pennsylvania, graduating in science in 1885 and entering Harvard University. After getting his Ph.D. in 1888, he spent a year in Germany where he studied under Victor Meyer etc. Back at Harvard he became Assistant in Chemistry, then Instructor, Assistant Professor and finally Professor in 1901. In 1903 he became Chairman of the Department of Chemistry at Harvard and in 1912 he was appointed Erving Professor of Chemistry and Director of the Wolcott Gibbs Memorial Laboratory .

About half of his original work concerned atomic weights, starting in 1886 with work on oxygen and copper. He invented the nephelometer and by 1912 he had redetermined, with the highest accuracy, the atomic weights of over thirty important elements and in later years he was to play his part, by his work on the determination of the atomic weight of isotopes, in the modern concept of the atom.

Professor Richards received honorary doctorates and honours from around the world. He was married with one daughter and two sons. His favourite recreations were sketching, golf and sailing. He died at Cambridge, Massachusetts, on 2 April, 1928.