| Tomonaga Sanjūrō | |
| Native Name: | 朝永 三十郎 |
| Other Names: | Sanjūrō Tomonaga |
| Birth Date: | 25 March 1871 |
| Birth Place: | Kawatana, Japan |
| Nationality: |  Japan |
| Occupation: | University professor |
| Children: | Shinichirō Tomonaga |
| Discipline: | Philosophy |
| Sub Discipline: | History of philosophy |
| School Tradition: | Kyoto School |
| Workplaces: | |
| Main Interests: | |
| Notable Students: | Obara Kuniyoshi |
was a Japanese academic and esteemed professor emeritus of medieval, renaissance, early modern, and Kantian philosophy at the University of Kyoto during the early 20th century. He was one of the leading thinkers of the Kyoto School.
His son, Shinichirō Tomonaga, is also renowned for receiving the 1965 Nobel Prize in Physics for the development of quantum electrodynamics.
Tomonaga was born in Nagasaki Prefecture the second son of Tomonaga Jinjirō, a samurai of the Ōmura Domain, in 1871. After graduating from Nagasaki Ōmura Junior High School (now known as Nagasaki Prefectural Omura High School) and then First Higher School, he entered the Tokyo Imperial University. After graduating, he became the assistant professor of philosophy at Kyoto Imperial University in 1907 and then full professor in 1913. He mainly lectured on Western philosophy and history of philosophy, and along with Nishida Kitarō and Tanabe Hajime, constituted the important intellectual Kyoto School movement of modern Japan.
Tomonaga was well known to be an unprolific writer but left a prestigious body of work and was mentor to many renowned Japanese philosophers, including Amano Teiyū, Obara Kuniyoshi, Yamauchi Tokuryū and Kosaka Masaaki. He retired from Kyoto Imperial University in 1931 and then became full professor at Ōtani University.