This article combines insights from historical institutionalism with the literature on ideational policy change to explain why Belgian sport institutions like the national football team appear as outliers in the story of the country's unidirectional move towards a decentralised federal state. It shows how Flemish nationalist ideas drove the asymmetrical decentralisation of the originally unitary Belgian sport system. On one hand, mechanisms of gradual institutional change led to the complete decentralisation of public sport policy. The Flemish government consequently has the authority and means to generate symbols and policies that boost Flemish identity through sport, while no comprehensive policies can be developed at the federal level for instrumentalising sport to build Belgian identity. On the other hand, mechanisms of institutional stability including vested interests, strong veto players and Flemings' dislike of secessionism prevent the full decentralisation of the Belgian non-governmental sport governing bodies. This implies that Belgium continues to be represented in international sport competitions as a nation and that sport can thus also be employed to create and sustain Belgian alongside Flemish identity. Sport therefore contributes to sustaining the Flemings' dual Flemish-Belgian identity, which ultimately stands in the way of a fully autonomous Flemish state.