Soshu Masahiro is a lineage of smiths working in Sagami Province that continued for four or five generations, traditionally transmitted as descending from the youngest son of Masamune. The first generation, styled "Soshu Masahiro," produced works bearing era names such as Joji and Meitoku, with the earliest known dated example inscribed Sadaharu 2 (1363). However, from the standpoint of chronology, it is not feasible to connect Masahiro and Masamune directly; rather, it is necessary to posit the existence of one additional intermediary figure between them. The line extended to the end of the Muromachi period, and it is also said that the fourth generation changed his name to Tsunahiro; however, the distinctions among the second and third generations are not clearly established.

The Masahiro line works in a Soshu-den idiom characterized by itame-hada forging, sometimes mixed with mokume, with abundant ji-nie and chikei. The hamon favors a notare-based composition mixing gunome and choji, enriched with well-entering ashi and yo, and accompanied throughout by sunagashi and kinsuji. In more vigorous examples, tobiyaki develops into an overall hitatsura-like manner, and the nioiguchi is bright and clear with ko-nie adhering well. The boshi typically enters in midare-komi, and the manner of cutting the inscription "Soshu-ju Masahiro" closely resembles the practice seen in Akihiro, indicating the shared technical milieu of late Soshu tradition. Early-generation works frequently display hira-zukuri construction with mitsu-mune, broad mihaba, thin kasane, and sun-nobi proportions with sakizori — the characteristic Nanbokucho-period form — and many bear accomplished horimono including bonji, kurikara, dokko-sho, and rendai.

The NBTHK consistently praises the Masahiro line as displaying the excellence of the Soshu tradition, with workmanship of both ji and ha rated as excellent and of upper-level quality. Well-preserved examples are noted as kenzen — sound in condition — and the carvings executed on the blades are repeatedly described as splendid. Works such as the tachi recorded in Nihonto Taikan show connections with Hiromitsu and Akihiro through their minayaki-like tempering, further attesting to the line's place within the broader circle of later Soshu-den masters who carried forward and sustained the legacy of the Masamune tradition through the Muromachi period.