Ceuta has been part of Spain since 1668, when the treaty of Lisbon was signed, putting an end to the war of the Portuguese Restoration that had started in 1640 with the proclamation of John of Bragança as king of Portugal. Up until then, Ceuta had been Portuguese, though between 1580 and 1640 it did not matter that much, since the crowns of Portugal and Spain were in a personal union, that it is to say they had the same person as king, but the laws, currency, etc were different.

When the war broke out in 1640, Ceuta broke out from Portugal and swore fealty to king Philip IV of Spain. By 1656, king Philip IV granted letters patent of naturalisation to Ceuta, formally incorporating all of the inhabitants of Ceuta to his dominions, and granted the city the title of "Fidelísima" (Most Faithful).

Prior to 1580, Ceuta was a Portuguese posession. In the first overseas adventure of the Portuguese crown (the first of many and glorious adventures), king John I, prince Henry the Navigator, prince Edward, and prince Peter crossed the Strait of Gibraltar with an army, and after a short battle they took the city, incorporating it to Portugal in 1415.

So, long story short: Ceuta is offically part of Spain since 1668, because the people of Ceuta so decided in 1656.