- Birth: 1546
- Death: 1610
- Distinction: Toyotomi retainer
- Title: Danjô
- Sons: Asano Yukinaga, Asano Nagaakira, and Asano Nagashige
Nagamasa was a son of Yasui Shigetsugu and a grandson of Asano Nagakatsu and was adopted into the Asano family. He later became the brother-in-law of Toyotomi Hideyoshi's wife and accompanied Hideyoshi on his campaigns in western Honshu (1577-82). He acted as a negotiator to Tokugawa Ieyasu following the Komaki Campaign (1584) and following his involvement in the Odawara Campaign (1590) was given a 200,000-koku fief at Fuchu (Kofu) in Kai province (1590). That same year he was tasked with carrying out land surveys in Dewa and Mutsu Provinces. He went on to serve in the Korean Campaigns, his time there highlighted by the long siege of Ulsan (1597-98) he endured along with Kato Kiyomasa. He was named one of the San-Bugyô in 1598 by Hideyoshi, though with his son Yukinaga he sided with Tokugawa Ieyasu during the Sekigahara Campaign (1600).