Nagasone Kotetsu (長曾禰虎徹) (c. 1597-1678) was a Japanese swordmaker of the early Edo period. Kotetsu was born in Sawayama around 1597, and was first known as Nagasone Okisato. His father was an armorer who served Ishida Mitsunari, the lord of Sawayama. However, as Ishida was defeated by Tokugawa Ieyasu at the Battle of Sekigahara, the Nagasone family and some other craftsmen from Sawayama went to Echizen province, where they took refuge in Fukui city.
Kotetsu worked as an armorer for some time, but later switched to swordmaking. His swords were known for their great strength and their ability to cut through helmets. However, Kotetsu's swords were often faked, and they were so well faked that even Kotetsu himself is said to have not been able to tell the difference. When presented with a fake on one occasion, he is reported to have said "The blade is mine but the signature is not." It is interesting to note that records exist which state that Nagasone Kotetsu only forged thirty one swords bearing the name.
Perhaps one of the most famed Kotetsu blades was a fake: that of Kondō Isami, the commander of the late Edo-era patrol force called Shinsengumi. However, this sword was not a Kotetsu, but instead a sword made by the foremost smith of that era (known in Japanese swordmaking history as the shinshin-to era), Minamoto Kiyomaro, and bearing a forged Kotetsu signature made by master signature-faker Hosoda Heijirō.
Nagasone Okisato took the name Kotetsu upon taking the Buddhist tonsure in Edo, at Kan'eiji Temple, in the Ueno district. He was active in the Kantō Region for some time, as well as in Edo itself, passing away in 1678. Two of his most prominent students and successors were Nagasone Okinao and Nagasone Okihisa.