St. Augustine was founded by Pedro Menéndez de Avilés in 1565 and is the oldest continuously occupied European settlement in the contiguous United States.
Menéndez established the Spanish settlement on September 8, 1565, naming it San Agustín after the feast day of Saint Augustine. Spain wanted a permanent base in Florida to protect its claims and help defend shipping routes connected with the Spanish treasure fleets.
The area was already inhabited by Timucua people before Spanish colonization. St. Augustine became the capital of Spanish Florida and remained an important administrative and military center through changing imperial control. Britain held Florida from 1763 to 1783, after which Spain regained it temporarily.
The city later became part of the United States after Spain ceded Florida in the Adams–Onís Treaty. The description applies to the contiguous United States, not all U.S. territory: San Juan, Puerto Rico, was founded earlier, in 1521.