Nagai Naomasa was a Japanese daimyō of the Edo period, who ruled the Uruido, Koga and Yodo Domains. The eldest son of Nagai Naokatsu, he fought at the Battle of Sekigahara and the Siege of Osaka. During the Shimabara Rebellion he was assigned to defend Kyoto.
Naomasa held junior 4th court rank, lower grade and the title of Shinano no Kami. He retired in early 1658 and became a monk, taking the name Shinsai.