Atlético Nacional S. A., best known as Atlético Nacional, is a Colombian professional football club based in Medellín. The club is one of only three clubs to have played in every first division tournament in the country's history, the other two being Millonarios and Santa Fe.

Atlético Nacional was founded in 1947 as Club Atlético Municipal de Medellín by Luis Alberto Villegas López, a former president of the football league of Antioquia. The team adopted its current name in 1950. The current owner, Organización Ardila Lülle, officially acquired the team in 1996. According to CONMEBOL, Atlético Nacional is the club with the largest fan following in Colombia, with more than 15 million fans.

Atlético Nacional plays its home games at the Atanasio Girardot stadium, which has a capacity of 40,043. They share the stadium with their local rivals, Independiente Medellín. The teams face each other in a local derby known as El Clásico Paisa, which is considered one of the most important local derbies in the country. Atlético Nacional also has rivalries with Millonarios and América de Cali, two of the most important rivalries in Colombian football and South America.

Considered to be one of the strongest clubs from Colombia, it is one of the most consistent clubs in the country. Nacional is the only Colombian club that has won the two domestic short-format tournaments in a single year, Apertura and Finalización, since the format was established in 2002, winning the titles of the 2007 and 2013 seasons. The club has won 18 league championships, seven Copa Colombia and four Superliga Colombiana titles, for a total of 29 domestic titles, making it the most successful team within Colombia. It was also the first Colombian club to win the Copa Libertadores in 1989 and, after winning the title again in 2016, the most successful Colombian side in that tournament. It also has the most international titles of any Colombian club, having also won the Copa Merconorte twice, the Copa Interamericana twice, and the Recopa Sudamericana once, for a total of seven international trophies and 36 overall.

In 2016, Atlético Nacional was ranked by IFFHS as the best football club in the world, becoming the first South American club, and the first outside Europe, to receive such an honor in that ranking. It ranks 58th in the world ranking of the best clubs of all time according to the IFFHS, being the second-best-ranked Colombian team on the list. It is also ranked as the second-best Colombian club of the 20th century and as the best Colombian club so far in the 21st century. Nacional is also credited as the best Colombian team in CONMEBOL club tournaments and ranks 17th in the official club ranking of the Copa Libertadores.

As of 1 January 2021, Atlético Nacional had 9.39 million followers on social media, making it the most-followed club in Colombia and one of the most-followed clubs in South America.

Atlético Nacional was founded as Club Atlético Municipal de Medellín on 7 March 1947 by a partnership led by Luis Alberto Villegas López, former president of the football league of Antioquia. The club was created to promote sports in the city, especially football and basketball. It was based on Unión Indulana Foot-Ball Club, an amateur club from the Liga Antioqueña de Fútbol, the local amateur football league. Officially, the founding members were: Luis Alberto Villegas Lopera, Jorge Osorio, Alberto Eastman, Jaime Restrepo, Gilberto Molina, Raúl Zapata Lotero, Jorge Gómez Jaramillo, Arturo Torres Posada and Julio Ortiz.

Atlético Nacional joined the professional league for its first edition in 1948. For that tournament, each club had to pay a fee of 1,000 pesos (at that time, approximately US$1,050). Atlético Nacional played the first match of the history of the tournament, a 2–0 victory over Universidad. The tournament had ten participants that season and Atlético Nacional was 6th with seven victories, four draws and seven defeats.