Nagai Nagayoshi (長井 長義, August 8, 1844 – February 10, 1929) was a Japanese doctor of pharmacy and chemist.

Nagai was born in Myōdō District, Awa Province in what is now Tokushima Prefecture, as the son of a doctor and started studying rangaku medicine at the Dutch Medical School of Nagasaki (Igaku-Denshujo) in 1864.

While in Nagasaki, he made the acquaintance of Ōkubo Toshimichi, Itō Hirobumi, and other future leaders of the Meiji government.

[citation needed] Nagai returned to Japan in 1883 to take up a position at the Tokyo Imperial University, and became Professor of Chemistry and Pharmacy there in 1893.

[1] His son, Alexander Nagai, served as a diplomat at the Embassy of Japan in Berlin until the end of World War II.