Football Club Internazionale Milano, commonly referred to as Internazionale (pronounced [internattsjoˈnaːle]) or simply Inter (Italian pronunciation: ['inter]), and colloquially known as Inter Milan in other countries, including English-speaking countries, is an Italian professional football club based in Milan, Lombardy. Inter is the only Italian team to have always participated in the top division of Italian football since its debut in 1909 and the only one that has never been relegated in Serie B. Since 1947, Inter has shared the San Siro stadium with AC Milan—the club from which it originally split. The San Siro is the largest stadium in Italy, with a capacity of 75,817. The long-standing rivalry between the two clubs, known as the Derby della Madonnina, is one of the most widely followed derbies in world football.

Founded in 1908 following a schism within the Milan Foot-Ball and Cricket Club (now AC Milan), Inter won its first championship in 1910. Since its formation, the club has won 37 domestic trophies, including 20 league titles, nine Coppa Italia, and eight Supercoppa Italiana. From 2006 to 2010, the club won five successive league titles, equaling the all-time record at that time. They have won the European Cup/Champions League three times, their latest win in 2010 completed an unprecedented Italian seasonal treble, with Inter winning the Serie A and the Coppa Italia the same year. The club has also won three UEFA Cups, two Intercontinental Cups, and one FIFA Club World Cup. Inter is the only Italian club that won at least an official trophy in every decade since the foundation of the club in 1908.

Inter has the highest home game attendance in Italy and the fourth-highest attendance in Europe. Since May 2024, the club has been owned by American asset management company Oaktree Capital Management.

The club was founded on 9 March 1908 as Football Club Internazionale, when a group of players left the Milan Cricket and Football Club (now AC Milan) to form a new club because they wanted to accept more foreign players. The name of the club derives from the wish of its founding members to accept foreign players as well as Italians. The club won its first championship in 1910 and its second in 1920. The captain and coach of the first championship winning team was Virgilio Fossati, who was later killed in battle while serving in the Italian army during World War I.

In 1922, Inter was at risk of relegation to the Second Division of Northern League, but they remained in the top league after winning two play-offs.

Six years later, during the Fascist era, the club merged with the Unione Sportiva Milanese and, for political reasons, was renamed Società Sportiva Ambrosiana. During the 1928–29 season, the team wore white jerseys with a red cross emblazoned on it; the jersey's design was inspired by the flag and coat of arms of the city of Milan. In 1929, the new club chairman Oreste Simonotti changed the club's name to Associazione Sportiva Ambrosiana and restored the previous black-and-blue jerseys; however, supporters continued to call the team Inter, and in 1931, new chairman Pozzani succumbed to shareholder pressure and changed the name to Associazione Sportiva Ambrosiana-Inter.

Inter won its third championship titles in 1930 with the Hungarian coach Arpad Weisz in the first ever edition of Serie A, and the fourth in 1938 with former player Armando Castellazzi as a 33 years old coach, that set the record for the youngest coach ever to win the national title that lasts to this day. Inter also got their first Coppa Italia (Italian Cup) in 1939 with the decisive goal in the final scored by Olympic gold medal and top scorer in 1936 Olympics Annibale Frossi. Inter main star and the captain of the team in this period was Giuseppe Meazza, one of the greatest Italian player of all time with two World Cup won with the National team and the greatest scorer in Inter history with 284 goals, and after whom the San Siro stadium is officially named after his death in 1980. 38 goals scored by Meazza in 39 matches in 1929-1930 is a seasonal record in Inter history still unbeaten today. Inter ended also three consecutive times in 2nd place between 1933 and 1935; in those years many South Americans of Italian origin arrived in Milan to circumvent the regime's rules that prohibit the hiring of foreign players: Uruguayan players like World Cup Winner in 1930 Hector Scarone and Ernesto Mascheroni and also Ricardo Faccio and Francesco Frione, and Argentinian like Attilio Demaría that stayed 10 seasons with the club. A fifth championship followed in 1940, that ended a decade dominated by three teams: Inter, Bologna and the historic rival Juventus, while AC Milan didn't win a title for 44 years from 1907 to 1951 and didn't win a single derby for a record 17 matches from 1928 to 1938.

In the 30's Inter also played for seven times in one of the first major European football cups, the Central European Cup, with Meazza that was a record three times topscorer of the competition; coached by Árpád Weisz Inter reached the final in 1933, when after had won the first leg in Milan 2–1, lost 3–1 in 9 men against Austria Vienna. 4 out of 11 players of that team: Meazza, Luigi Allemandi, Attilio Demaría and Armando Castellazzi would go on to win the 1934 World Cup with Italian national team, while other five Inter players will contribute to the win of 1938 World Cup with Italy: Meazza, Ugo Locatelli, Giovanni Ferrari, Pietro Ferraris and Renato Olmi.