The idea that the preservation of competition through competition law contributes to the protection of democracy constitutes a recurrent theme, or even a foundational myth, of US antitrust and EU competition law. Yet, legal scholarship has so far failed to provide a coherent explanation as to why the preservation of competitive markets and the control of private economic power are important for democracy. This study purports to unpack this idea of a competition-democracy nexus and to put forward a clear answer to the question of how competition and competition law promote and protect democracy.   The primary claim of this study is that the idea of a competition-democracy nexus can only be explained by the republican concept of liberty as non-domination that originated from republican thought in Ancient Rome. This republican concept of liberty differs from our predominant understanding of negative liberty which perceives only the actual or likely interference by somebody else with our choices or actions as a source of unfreedom. Instead, republican liberty defines liberty in opposition to a master-slave relationship. It considers the mere presence of and defenceless subjugation to the arbitrary power and domination of another person as an obstacle to individual liberty, even if this person does not interfere with our choices. This republican concept of liberty can, thus, explain the basic assumption underlying the idea of a competition-democracy nexus that the mere existence of concentrated economic power is in itself incompatible with a republican democracy and a society of free and equals, in spite of the absence of any concrete risk of interference.   Using the concept of republican liberty as the explanatory variable for the idea of a competitiondemocracy nexus, this study makes four major contributions. First, it traces the historical trajectory of the idea that competition promotes democracy. It shows that early proponents of competitive markets, such as Adam Smith, as well as various antitrust movements in the US, and the Ordoliberal School in Europe shared the common belief that competition by diffusing economic power operates as an institution of ‘antipower’ that promotes republican liberty and democracy. Second, this study also explores how US and EU competition law have operationalised this concern about republican liberty and democracy by protecting a polycentric market structure in which power is dispersed amongst many independent decision-makers. Third, it sheds light on how the rise of the Chicago School in the US and the More Economic Approach in Europe have displaced the concern about the competition-democracy nexus and republican liberty with a negative understanding of liberty that only perceives welfaredecreasing interference as an obstacle to economic liberty. Fourth, in light of growing societal concerns about the concentration of corporate power, this study also signposts some of the parameters which would have to be recalibrated in order to realign competition law with a republican understanding of economic liberty and to reinvigorate the link between competition and democracy.