The War of the Castilian Succession is the military conflict contested from 1475 to 1479 for the succession of the Crown of Castile fought between the supporters of Joanna la Beltraneja, daughter of the late monarch Henry IV of Castile, and those of Henry's half-sister, Isabella, who is ultimately successful.The war has a marked international character, as Isabella is married to Ferdinand, heir to the Crown of Aragon, while Joanna is strategically married to King Afonso V of Portugal, her uncle, after the suggestion of her supporters.

France intervenes in support of Portugal, as they are rivals with Aragon for territory in Italy and Roussillon.Despite a few initial successes by the supporters of Joanna, a lack of military aggressiveness by Afonso V and the stalemate in the Battle of Toro (1476) leads to the disintegration of Joanna's alliance and the recognition of Isabella in the Courts of Madrigal-Segovia (April–October 1476).The war between Castile and Portugal alone continues.

This includes naval warfare in the Atlantic, which becomse more important: a struggle for maritime access to the wealth of Guinea (gold and slaves).

In 1478, the Portuguese navy defeats the Castilians in the decisive Battle of Guinea.

The war concludes in 1479 with the Treaty of Alcáçovas, which recognizes Isabella and Ferdinand as sovereigns of Castile and grants Portugal hegemony in the Atlantic, with the exception of the Canary Islands.

Joanna loses her right to the throne of Castile and remains in Portugal until her death.This conflict has also been called the Second Castilian Civil War, but this name may lead to confusion with the other civil wars that involve Castile in the fourteenth and fifteenth centuries.

Some authors refer to it as the War of Portugal; however, this name clearly represents a Castilian point of view and implicitly denies Juana's claim.

At other times the term Peninsular War has been used, but it is easily confused with the Peninsular War of 1808–1814, part of the Napoleonic Wars.

Some authors prefer the neutral expression War of 1475–1479.