Final answer:
El Salvador officially gained independence from Spain in 1821, as part of the broader Central American independence from colonial rule.
Step-by-step explanation:
El Salvador gained independence from Spain in the context of a broader Latin American movement towards independence. Central American republics, including El Salvador, declared their independence from Spanish rule after a series of events that destabilized Spanish authority in the region. El Salvador officially achieved independence in 1821 following the adoption of the Plan de Iguala and the formation of the Army of the Three Guarantees, which resulted in Mexico's independence. Subsequently, the Act of Independence of Central America was signed on September 15, 1821, which included El Salvador among other Central American territories, marking their collective independence from Spain.