Tomonaga is a Japanese surname. Notable people with the surname include:
Jacobo Kyushei Tomonaga (1582–1633), Japanese priest in the Dominican Order of the Catholic ChurchMinamoto no Tomonaga (1144–1160), Minamoto clan samurai of the late Heian periodNatsumi Tomonaga (born 1991), Japanese modern pentathleteSin-Itiro Tomonaga (1906–1979), Japanese physicist and Nobel Prize winner, influential in the development of quantum electrodynamics