Azai Nagamasa (浅井 長政, 1545 – 26 September 1573) was a Japanese daimyō of the Sengoku period known as the brother-in-law and enemy of Oda Nobunaga.

[1] 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.

Many retainers wished to honor the alliance with the Asakura, while Nagamasa himself is reported to have favoured staying neutral, essentially siding with Nobunaga.

[3]: 60–61 In the summer of 1570, Oda Nobunaga and Tokugawa Ieyasu brought an army estimated between 20,000-30,000 men into Northern Omi.

Over the course of the next two years, with the exception of occasional interventions by the Shogun, Azai Nagamasa was under constant threat of Nobunaga aggression into Omi.

During this period the Azai are seen as being loosely aligned with numerous anti-Oda forces, including the Asakura, the Miyoshi, the Rokkaku, and several religious complexes.

[3]: 65 [4] Nobunaga later convinced Oichi to tell him where she had sent her infant son and Nagamasa's male heir, Manpukumaru, saying that he wanted the boy to live with and raise him.

There are reports that Nobunaga bore a strong grudge against Nagamasa for his perceived betrayal of their alliance even though it was he who broke the agreement first.