Mogami Yoshiaki History Museum

The Mogami Yoshiaki History Museum stands near Kajo Park (the former site of Yamagata Castle) in Yamagata City's Otemachi district. It centers on artifacts related to Mogami Yoshiaki, the Sengoku-era lord who laid the foundations of the Yamagata domain, preserving armor, swords, historical documents, and paintings connected to the Mogami family.

Its core holdings include the "38-plate gold-rimmed helmet" said to have been worn by Yoshiaki at the 1600 Battle of Hasedo, along with other armor pieces and personal effects, a folding screen depicting the battle, and sliding-door paintings once housed in Yamagata Castle's main keep. The museum periodically holds themed exhibitions spotlighting swords and armor, including works by swordsmiths connected to the former Dewa Province and contemporary smiths.

Admission has been free since April 2009. The museum closes Mondays (opening instead on national holidays, with the following weekday closed), during exhibition changeovers, and over the New Year period. Parking is available at nearby Otemachi, Kajo Park, and Sakuramachi lots.

Mogami Yoshiaki, uncle of Date Masamune, ruled the Yamagata domain and is remembered for repelling Uesugi forces at the Battle of Hasedo — one of Tohoku's largest Sengoku-era battles — while laying the groundwork for Yamagata's castle town.