The King Baudouin Stadium (French: Stade Roi Baudouin [stad ʁwa bodwɛ̃]; Dutch: Koning Boudewijnstadion [ˌkoːnɪŋ ˈbʌudəʋɛinˌstaːdijɔn]) is a sports ground in Brussels, Belgium. Located in the north-western district of the City of Brussels, it was built to embellish the Heysel/Heizel Plateau in view of the 1935 Brussels International Exposition. It was inaugurated on 23 August 1930, with Crown Prince Leopold attending the opening ceremony. The stadium hosted 70,000 at the time. Its name honours King Baudouin, Leopold's successor as King of the Belgians from 1951 to his death in 1993.

The stadium is located at 135/2, avenue de Marathon/Marathonlaan, on the border of the Bruparck entertainment park (with the Atomium, Mini-Europe miniature park and Kinepolis cinema). It can be accessed from the metro stations Heysel/Heizel and Roi Baudouin/Koning Boudewijn on line 6.

The first version of the King Baudouin Stadium was built in 1929–30 by the architect Joseph Van Neck, also chief architect of the 1935 Brussels International Exposition, in a classical modernist style. Its original name was the Jubilee Stadium (French: Stade du Centenaire, Dutch: Jubelstadion) because it was inaugurated as part of the centenary celebrations of the Belgian Revolution, with an unofficial Belgium–Netherlands football match.

In 1946, the stadium was stripped of the wood of its cycling track, and was renamed the Heysel Stadium (French: Stade du Heysel, Dutch: Heizelstadion), after the neighbourhood in which it is located. In 1971, a tartan track was installed allowing the organisation of athletics competitions. Three years later, in 1974, a new lighting system was installed.

The Heysel Stadium hosted European Cup finals in 1958, 1966, 1974, and 1985 and Cup Winners' Cup finals in 1964, 1976 and 1980. The highest attendance at a European game was over 69,000 in 1958.

Despite its status as Belgium's national stadium, the Heysel Stadium was not well maintained. The stadium's poor condition manifested itself at the 1985 European Cup Final. For example, the outer wall had been made of cinder block, and fans who did not have tickets were seen kicking holes in it to get in. Additionally, the only escape route led upward, and there were only three gates on each short side–nowhere near enough for the 22,000 people standing on the terraces on either side.

The stadium's inadequacies had been well known for some time. When Arsenal played there in the early 1980s, its supporters ridiculed it as a "dump." Indeed, the presidents of the two 1985 European finalists, Juventus and Liverpool, had concluded that Heysel was in no condition to host a European Final, especially one featuring two of the largest and most powerful clubs in Europe at the time. They urged UEFA to move the match to another ground, to no avail. It later emerged that UEFA had only spent half an hour inspecting the stadium.

On 29 May 1985, the Heysel Stadium disaster occurred, resulting in the deaths of 39 Juventus spectators in a crowd disorder caused by Liverpool fans before the match. Despite the disaster, the stadium continued to be used for Belgium international games from 1986 to 1990 with only minimal improvements made following the disaster. This was in part because the government had already drawn up plans to remodel the stadium into a 35,000-seat facility. Finally, in 1990, UEFA forced the issue by barring Belgium from hosting a European Final until at least 2000. It also continued to host track and field events and it still hosts the Memorial Van Damme every year.