Yoshisada (左吉貞) was a swordsmith of the ichirui, the group of makers who inherited the tradition of , also known as , of Province. He is traditionally said to have been either a son of or a direct disciple within his circle. Because there exists a bearing the date Shohei 13 (1358) with the inscription "Shu Nagamasa / Yoshisada ," his approximate period of activity can be ascertained to the mid- era. The school emerged in the early period, departing decisively from the earlier, classical manner of Kyushu swordmaking to establish a refined style in which both and appear bright and clear, with conspicuous activity drawing upon the current of -. Within this lineage, prominent smiths including Yasuyoshi, Yukihiro, Yoshisada, Kunihiro, Hiroyuki, Hiroyasu, and Sadayoshi each inherited the master's approach while demonstrating excellent individual technique. Extant signed works by Yoshisada are chiefly and , and examples in form are exceedingly few. His signatures are most often cut simply as "Yoshisada" or "Yoshisada "; examples appending "Chikushu ju" are not known. One rare bears the four-character inscription "Mononobe Yoshisada," a prefix of exceptional documentary importance.
In forging, Yoshisada's works characteristically display mixed with and , with an overall tendency toward (standing grain). Thickly applied adheres with abundant , and the steel color tends toward a somewhat dark, blackish cast. Faint is sometimes observed. The is the primary vehicle of his individual manner within the group: where other members may produce more flamboyant temper patterns, Yoshisada's work tends toward a calm and restrained character, based on or mixed with , , and pointed elements, forming an overall small-patterned design. adheres well throughout, with fine and running vigorously; and may appear near the , and interweave with the tempered edge to produce varied and engaging visual effects. The typically shows a -like tendency, becoming pointed at the tip with , and the turnback is often long. In certain works the is tempered high, introducing a brilliant passage that contrasts with the otherwise subdued composition, while extended and thick overlaying the further demonstrate the depth and subtlety of his technique.
Within the ichirui, many works feature small-patterned , yet it is precisely this characteristic that most reliably distinguishes Yoshisada's hand from those of his contemporaries; even among unsigned blades attributed to the group, pieces displaying this restrained tendency are most plausibly assigned to him. His works consistently exhibit powerful with abundant interior activity, and the finest examples show both and in a (sound and well-preserved) state, with robust form and ample . Several blades of notable provenance survive, including pieces transmitted in the Ikeda family, the Mori family, and the Mito Tokugawa house -- one being a relic of Tokugawa Ieyasu himself, later distributed to Tokugawa Yorifusa of Mito. A recorded in the Kozan bearing the rare "Mononobe" prefix is considered exceedingly valuable as documentary material for research on the smith. Although his surviving corpus is modest in number, the quality of workmanship and the historical significance of his signed examples secure Yoshisada's position as one of the distinguished masters of the school of .