New Spain, formally called the Viceroyalty of New Spain (Spanish: Virreinato de Nueva España), was a viceroyalty of the crown of Castile. It was formed in 1535, as the realm of the Spanish empire which comprised the territories in the north overseas ‘Septentrion’, from North America and the Caribbean, to the Philippines.
New Spain was established following the Spanish conquest of the Aztec Empire in 1521. At its greatest extent, it included, on the mainland of the Americas, much of North America south of Canada, that is: all of present-day Mexico and Central America except Panama; most of theUnited States west of the Mississippi River, plus the Floridas.
To the west of the continent, New Spain also included the Spanish East Indies (the Philippine Islands, the Mariana Islands, the Caroline Islands, parts of Taiwan, and parts of the Moluccas) To the east of the continent, it included the Spanish West Indies (Cuba, Hispaniola(comprising the modern states of Haiti and the Dominican Republic), Puerto Rico, Jamaica, the Cayman Islands, Trinidad, and the Bay Islands).
On the mainland, the administrative units included Las Californias, that is, Alta California (present-day Arizona, California, Nevada, Utah, western Colorado, and south Wyoming); and Baja California Norte and Sur, Nueva Extremadura (the present-day states of Coahuila andTexas), and Santa Fe de Nuevo México (parts of Texas and New Mexico).  and Louisiana (including the western Mississippi river basin and the Missouri River basin).
The Spanish governed New Spain from Mexico City, formerly Tenochtitlan: the conquered capital of the Aztec Empire. It was ruled by aviceroy, governing the various territories on behalf of the King of Spain.
It was the first of four viceroyalties created to govern Spain’s overseas colonies. The Viceroyalty of Peru was created soon after, following theSpanish conquest of the Inca Empire in 1542. For nearly two centuries these were the sole viceroyalties, until in the 18th century theViceroyalty of New Granada, and the Viceroyalty of the Río de la Plata were also created.