Azai Nagamasa was a Japanese daimyō of the Sengoku period known as the brother-in-law and enemy of Oda Nobunaga. Nagamasa was head of the Azai clan seated at Odani Castle in northern Ōmi Province and married Nobunaga's sister Oichi in 1564, fathering her three daughters – Yodo-dono, Ohatsu, and Oeyo – who became prominent figures in their own right.

The Sengoku period, also known as Sengoku Jidai is the period in Japanese history in which civil wars and social upheavals took place almost continuously in the 15th and 16th centuries. Although the Kyōtoku incident (1454), Ōnin War (1467) or Meiō incident (1493) are generally chosen as the Sengoku period's start date, there are many competing historiographies for its end date, ranging from 1568, the date of Oda Nobunaga's march on Kyoto, to the suppression of the Shimabara Rebellion in 1638, deep into what is traditionally considered the Edo period. Regardless of the dates chosen, the Sengoku period overlaps substantially with the Muromachi period (1336–1573).