- Birth: 1846/1/31
- Death: 1903/6/17
- Original name: Ishii Sadayuki (石井貞行)
- Other name: Genjirô (源次郎)
- Japanese: 石井 勇次郎 (Ishii Yuujirou)
Ishii Yujiro was from Kuwana han in Ise province, and was the oldest son of Ishii Fumiya. He was a student of Jiki Shin-Kage Ryu. He joined the Shinsengumi in 1868 during the Boshin War. After the Battle of Hakodate, he was sent to Kuwana han and released in a year. He became a Mie prefecture staff.
- Shinsengumi - Honor and Determination of the Mibu Rôshi (血誠新撰組-峻烈壬生浪士忠と斬), Rekishi Gunzô series #31, Gakken, Japan, 2003
- Shinsengumi Zentaishi Tettei Guide (新選組全隊士徹底ガイド) Maeda Masaki