The Battle of Diu was a naval battle fought on 3 February 1509 in the Arabian Sea, in the port of Diu, India, between the Portuguese Empire and a joint fleet of the Sultan of Gujarat, the Mamlûk Burji Sultanate of Egypt, and the Zamorin of Calicut with support of the Republic of Venice and the Ottoman Empire.

The Portuguese victory was critical: the great Muslim alliance was soundly defeated, easing the Portuguese strategy of controlling the Indian Ocean to route trade down the Cape of Good Hope, circumventing the historical spice trade controlled by the Arabs and the Venetians through the Red Sea and Persian Gulf. After the battle, Kingdom of Portugal rapidly captured several key ports in the Indian Ocean including Goa, Ceylon, Malacca, Bom Baim and Ormuz. The territorial losses crippled the Mamluk Sultanate and the Gujarat Sultanate. The battle catalpulted the growth of the Portuguese Empire and established its political dominance for more than a century. Portuguese power in the East would begin to decline with the sackings of Goa and Bombay-Bassein, Portuguese Restoration War and the Dutch colonisation of Ceylon.

The Battle of Diu was a battle of annihilation similar to the Battle of Lepanto and the Battle of Trafalgar, and one of the most important in world naval history, for it marks the beginning of European dominance over Asian seas that would last until the Second World War.