Rosarito is a coastal city in Playas de Rosarito Municipality, Baja California, on the Pacific Coast of Mexico. As of 2010, the city had a population of 65,278. Located 10 miles (16 km) south of the US–Mexico border, Rosarito is a part of the greater San Diego–Tijuana region and one of the westernmost cities in Mexico. Rosarito is a major tourist destination, known for its beaches, resorts, and events like Baja Beach Fest.

Evidence of the presence of Paleo-Indians in the region has been dated as early as 2,000 BC. By 1,000 BC, a group emerged that is recognizable as the Yuman ancestors of the Kumeyaay, who continued to inhabit the northern portion of the Baja California Peninsula at the time of European contact. The Kumeyaay referred to the area now known as Rosarito as Wa-cuatay, which translates to "big houses" in the Kumeyaay language.

After conquering the Aztec Empire, Hernán Cortés sent expeditions to explore what he believed to be the Island of California. In 1533, mutineer Fortún Ximénez was the first European to land in Baja California, at La Paz, Baja California Sur. In September 1542, Juan Rodríguez Cabrillo passed through the Rosarito area on his way from Ensenada to San Diego Bay. The year of 1697 saw the establishment of the first permanent European settlement in Baja California in a Jesuit mission at Loreto. Rosarito would soon be caught in a power struggle between Jesuits, Dominicans, and Franciscan friars for decades.

In 1773, a frontier was defined separating Nueva ("new") or Alta ("upper") California, under the jurisdiction of the Franciscans, from Antigua ("old") or Baja ("lower") California, which was entrusted to the Dominicans. In 1788, Luis de Sales, a Dominican priest, redrew the boundary, extending Baja California to the Rosarito Arroyo, known at the time as the Barrabas Arroyo.

Because Mission San Miguel, 13 km (8.1 mi) to the south, was frequently flooded by the Río Guadalupe, Dominican missionary Tomás de Ahumada sought another site on higher ground for at El Descanso. Additional advantages of the northerly location were good grazing and plentiful arable land. In 1817, Ahumada founded the Misión El Descanso or Misión San Miguel la Nueva among the Kumeyaay people 22 km (14 mi) to the south of the present-day Rosarito.

The property of Rancho El Rosario, granted by José María de Echeandía in 1827 to Don José Manuel Machado, one of the first soldiers stationed at the Presidio of San Diego, was the first ranch in the modern-day Rosarito region. The 11 league rancho was bounded on the north by Rancho Tía Juana, on the west by the Pacific Ocean and on the south by public lands.

In 1830, Father Félix Caballero constructed an adobe church here and managed both missions from El Descanso, but in 1834, it ceased to function as a mission. At the time it had a population of just 254, including those from Mission San Miguel. By 1853 the mission was deserted. Ruins of Misión El Descanso remain within the community of El Descanso.

José Manuel Machado's son, Don Joaquín Machado, applied for title to the land as Rancho Rosarito to President Porfirio Díaz. On May 14, 1885, Machado received his title and registered it in Ensenada, then the capital city of the Northern District of Baja California Territory. May 14 is now recognized and celebrated as Rosarito's Foundation Day by the Historical Society of Rosarito.