Thomas Arthur Steitz (August 23, 1940 – October 9, 2018) was an American biochemist, a Sterling Professor of Molecular Biophysics and Biochemistry at Yale University, and investigator at the Howard Hughes Medical Institute, best known for his pioneering work on the ribosome.

Steitz was awarded the 2009 Nobel Prize in Chemistry along with Venkatraman Ramakrishnan and Ada Yonath "for studies of the structure and function of the ribosome".

Steitz also won the Gairdner International Award in 2007 "for his studies on the structure and function of the ribosome which showed that the peptidyl transferase was an RNA catalyzed reaction, and for revealing the mechanism of inhibition of this function by antibiotics".

Born in Milwaukee, Wisconsin, Steitz studied chemistry as an undergraduate at Lawrence University in Appleton, Wisconsin, graduating in 1962.

In June 2010, the University renamed its chemistry building Thomas A. Steitz Hall of Science.

Both Tom and Joan Steitz joined the Yale faculty in 1970, where he continued to work on cellular and structural biology.

Steitz and Peter Moore determined the atomic structure of the large 50S ribosomal subunit using X-ray crystallography, and published their findings in Science in 2000.

In 2009, Steitz was awarded the Nobel Prize in Chemistry for his ribosome research.

He was also a Macy Fellow at the University of Göttingen during 1976–1977 and a Fairchild Scholar at the California Institute of Technology during 1984-1985.Steitz was also one of the founders of a company, Rib-X Pharmaceuticals, now Melinta Therapeutics for the development of new antibiotics based on the ribosome.

He died on October 9, 2018 of complications during treatment of pancreatic cancer.