Inter is the most supported football club in Italy according to an August 2007 research by Italian newspaper La Repubblica. Historically the largest section of Inter fans from the city of Milan, have been the middle-class bourgeoisie Milanese, while AC Milan fans are typically working-class and a significant portion are migrants from Southern Italy.

The traditional ultras group of Inter is Boys San, they hold a significant place in the history of the ultras scene in general due to the fact that they are one of the oldest; founded in 1969. Politically the ultras of Inter are usually considered right-wing, as thus they have good relationships with Verona and Lazio. As well as the main group Boys San, there are four more significant groups; Viking, Irriducibili, Ultras, and Brianza Alcoolica.
Inter fans celebrating in 2007.

Inter have several rivalries, two of which are highly significant in Italian football; firstly they participate in the inter-city Derby della Madonnina with AC Milan, the rivalry has existed ever since Inter splintered off from Milan in 1908. The name of the derby refers to the Blessed Virgin Mary, whose statue atop the Milan Cathedral is one of the city's main attractions. The match usually creates a lively atmosphere, with numerous (often humorous or offensive) banners unfolded before the match. Flares are commonly present, but they also led to the abandonment of the second leg of the 2005 Champions League quarter-final matchup between Milan and Inter on 12 April, after a flare thrown from the crowd by an Inter supporter struck Milan keeper Dida on the shoulder.

The other most significant rivalry is with Juventus, the two participate in the Derby d'Italia. Up until the 2006 Serie A match-fixing scandal, which saw Juventus relegated, the two were the only Italian clubs to have never played below Serie A. Notably the two sides are the most supported in Italy, the rivalry has intensified since the later part of the 1990s. Clubs such as Bologna, Atalanta and Roma are also considered to be amongst their rivals.