Inagaki Manjirō (稲垣 満次郎, September 26, 1861 – November 25, 1908) was a Japanese diplomat and political theorist who was active during the Meiji period of Japan.

After studying at the clan-established Ishinkan and Kagoshima Shigakko (private school), he entered the Department of Literature of the Tokyo Imperial University in 1882.

He became a very popular figure at the University, especially with the Master of Pembroke College and the Vice-Chancellor, the Reverend Dr. Charles Edward Searle.

After graduating Inagaki returned to Japan, and became a temporary professor at Gakushuin Higher Commercial School.

Inagaki wrote a number of scholarly books in English and Japanese on international affairs but died relatively young with his potential unfulfilled.