A tozama daimyō (外様大名 , "outside daimyō") was a daimyō who was considered an outsider by the rulers of Japan. The term came into use in the Kamakura period and continued until the end of the Edo period.
The daimyō who submitted to the Tokugawa shogunate after the Battle of Sekigahara—who became Tokugawa vassals only after the battle—were classified as tozama. They included both daimyō who fought with the Tokugawa and those who fought against them. Many of the largest fiefs were ruled by tozama. The biggest was the Maeda clan of Kaga with a value of 1,000,000 koku. Others included the Shimazu clan of Satsuma, the Mori, the Date, Hachisuka, and the Uesugi. Many, but not all, of these families, had been living in roughly the same regions for centuries before the Tokugawa shogunate.
Tokugawa Ieyasu had treated the great tozama vassals amicably but later, between 1623 and 1626, Tokugawa Iemitsu was less tolerant of them. Particularly in western Japan, the tozama daimyō heavily profited from foreign trade in the mid 17th century. Their growing success was a threat to the shogunate, which responded by preventing the ports of western Japan and Kyūshū from trading.
To keep the tozama in check, the shogunate stationed fudai daimyō in strategic locations, including along major roads and near important cities. For much of the Edo period, the shogunate ordinarily did not appoint tozama to high positions within the government. These went instead to the fudai daimyō. However, this began to change in the Bakumatsu era; one tozama daimyō (Matsumae Takahiro) even became a rōjū.
Tozama daimyō from Satsuma and Chōshū (Shimazu and Mori clans respectively) were responsible for the fall of the Tokugawa shogunate during the Bakumatsu era. Rallying other tozama to their cause, they fought against the shogunate, Aizu, and the Ōuetsu Reppan Dōmei during the Boshin War of 1868–69. Many people from Satsuma and Chōshū dominated politics in the ensuing decades, and well into the 20th century, as part of the Meiji oligarchy.