Since his uncle, Nagakatsu ASANO, who was the head of Nobunaga ODA's archers, did not have a son, Nagamasa was adopted into the ASANO family as the husband of Nagakatsu's daughter, Yaya, becoming the heir.
織田信長の弓衆をしていた叔父の浅野長勝に男子がなかったため、長勝の娘のややの婿養子として浅野家に迎えられこれを相続した。