Tom Mike Apostol (/əˈpɑːsəl/ ə-POSS-əl;^[1] August 20, 1923 – May 8, 2016)^[2] was an American analytic number theorist and professor at the California Institute of Technology, best known as the author of widely used mathematical textbooks.

Apostol was born in Helper, Utah. His parents, Emmanouil Apostolopoulos and Efrosini Papathanasopoulos, were Greek immigrants.^[3] Apostolopoulos's name was shortened to Mike Apostol when he obtained his United States citizenship, and Tom Apostol inherited this Americanized surname.^[3]

Apostol received his Bachelor of Science in chemical engineering in 1944, Master's degree in mathematics from the University of Washington in 1946, and a PhD in mathematics from the University of California, Berkeley in 1948.^[4] Thereafter Apostol was a faculty member at UC Berkeley, MIT, and Caltech. He was the author of several influential graduate and undergraduate level textbooks.

Apostol was the creator and project director for Project MATHEMATICS! producing videos which explore basic topics in high school mathematics. He helped popularize the visual calculus devised by Mamikon Mnatsakanian with whom he also wrote a number of papers, many of which appeared in the American Mathematical Monthly. Apostol also provided academic content for an acclaimed video lecture series on introductory physics, The Mechanical Universe.

In 2001, Apostol was elected in the Academy of Athens.^[5][6] He received a Lester R. Ford Award in 2005,^[7][8][9] in 2008,^[10] and in 2010.^[11] In 2012 he became a fellow of the American Mathematical Society.^[12]