Alajuela (Spanish pronunciation: [alaˈxwela]) is a district in the Alajuela canton of the Alajuela Province of Costa Rica. As the seat of the Municipality of Alajuela canton, it is awarded the status of city. By virtue of being the city of the first canton of the province, it is also the capital of the Province of Alajuela.

Because of its location in the Costa Rican Central Valley, Alajuela is nowadays part of the conurbation of the Greater Metropolitan Area. The city is the birthplace of Juan Santamaría, the national hero of Costa Rica and the figure who gives the name to the country's main international airport, which is south of Alajuela downtown.

Alajuela has an area of 8.89 km² (3.43 sq mi) and an elevation of 952 metres (3,123 ft). It is located in the Central Valley, 19 kilometres northwest of San José.

The climate is tropical, typical of the Central Valley, but slightly warmer than San José. Temperatures are moderate, averaging 23–26 degrees Celsius (73–79 degrees Fahrenheit) with a low humidity level, with dewpoints around 20 (68 °F) almost all year round. Alajuela and its surroundings are famed for having "the best weather in the world".

For the 2011 census, Alajuela had a population of 42,975 inhabitants.

In pre-Columbian times the land where the canton of Alajuela is today was part of the Western Huetar Kingdom, which was inhabited by native tribes, who at the time of the Spanish conquest were led by Chief Garabito.

The first Spanish settlers established settlements in the region in about 1650. In a letter of obligation granted in 1764, the place is mentioned as La Lajuela in the Valley of Barva, near the Canoas river.

In 1777, the dwellers of La Lajuela and Ciruelas, having been served with notice to move to Villa Vieja (today's Heredia), requested the provisional construction of a public place of prayer in the house of Don Dionysius Oconitrillo, of Spanish origin, 30 metres north of where Alajuela's cathedral is today.