Mizoguchi Naomasa (溝口直正, April 11, 1855 – October 17, 1919) was the 12th (and final) daimyō of Shibata Domain in Echigo Province, Japan (modern-day Niigata Prefecture).

That year, all of the daimyō were called to Kyoto by Shōgun Tokugawa Yoshinobu, but due to his youth, he sent the karō of the domain in his place.

Shibata was unable to make any military contribution to the alliance due an uprising of the peasantry.

Yonezawa Domain dispatched troops to Shibata and attempted to take Mizoguchi Naomasa hostage to ensure the domain's cooperation to the Alliance, but the effort failed due to strong local opposition.

Enough of the domain's troops joined the Alliance to prevent a further attack; however, these actions prompted Naomasa to immediately defect to the Imperial forces when they landed at Kashiwazaki, and Shibata Castle subsequently became a stronghold for government forces against the Alliance for the remainder of the Boshin War In 1868, Naomasa was received in audience by Emperor Meiji, and from 1869 to 1871 served as imperial governor of Shibata under the new Meiji government.