相馬氏
was the 1st "daimyō" of Sōma-Nakamura Domain and the 17th hereditary chieftain of the Sōma clan. He was the eldest son of Sōma Yoshitane, the 16th Sōma chieftain. His courtesy title was "Daizen-no-suke" and Court rank was Junior Fifth Rank, Lower Grade. During the Battle of Sekigahara, he attempted to remain neutral, as his father was on close terms with Ishida Mitsunari and as he had concerns with regards to his powerful neighbors, the Satake clan. However, the forces of Tokugawa Ieyasu won the battle, and with the establishment of the Tokugawa shogunate, the clan faced the loss of their hereditary domains. The shogunate relocated the Satake to Akita Domain and despite a considerable loss of "kokudaka", the Satake offered to set aside a portion of their new territories for the Sōma clan. However, Sōma Toshitane (at that time named Sōma Mitsutane) decided to petition the shogunate for reinstatement instead, and changed the "kanji" of his name from 三胤 to蜜胤 to remove the character in his name that he had shared with Ishida Mitsunari. A number of senior Tokugawa retainers spoke out on his behalf, including Honda Masanobu, and the Sōma clan's traditional rival, Date Masamune. The shogunate agreed to his petition, and the Sōma clan were allowed to keep their lands.