In a time characterised by the increasing erosion of democratic institutions, processes of depoliticisation, and the imposition of a consensual order, this PhD thesis explores different modes of engagement with cultural institutions, focusing specifically on instituting practices as a model for the reinvention of democratic politics. Building on contemporary political philosophy, post-structuralist theory, and cultural studies, it begins by analysing both dissensus, as the democratic principle that entails the never-ending redefinition of the members of the society (meaning, whose voice is heard and recognised as a legitimate partner in the debate), and antagonism, as the founding moment and constitutive element that grounds the social (in the form of the exclusions that have been eventually naturalised). It demonstrates that dissensus is an inherently aesthetico-political phenomenon that has very distinct performative and spatial characteristics, since it involves the staging of equality, the introduction of new radical imaginaries, and the recalibration of the aesthetic register. Although dissensus is considered a fleeting moment, a rapturous incident, and a disruptive event with no 'proper' place, I set out to conceptualise the possibility of instituting dissensus, claiming antagonism, and channeling them towards the democratisation of cultural institutions. This entails the attempt to envisage an open, dynamic, and self-reflective model of a "dissensual institutionality" that can safeguard the continuous inscription of a multiplicity of social demands. Drawing on my ethnographic research in Europe and the U.S., this theoretical construct is then tested out through a series of under-researched empirical cases. The examined case studies include attempts of decentering and deterritorialising hegemonic machines and mega-institutions (such as documenta 14 in Athens, which foregrounded decolonial narratives and dissident histories from the periphery); endeavours of inventing new flexible organisational arrangements (such as alter-institutions in Athens, Paris, and Bochum that introduced novel parliamentary formats and decision-making processes); and, finally, modes of critical engagement with solidified institutional structures that set out to seize, reform, or even dismantle them (such as artistic activist initiatives in the U.S. that center-staged questions of toxic philanthropy, museum sponsorship, and labour rights). My investigation of 'instituting dissensus' ends by drawing some useful conclusions on the possibilities and limitations of the respective strategies, methodologies, and practices that take place within, at the threshold of, or outside institutional structures, in the attempt to enact new egalitarian political imaginaries.