The goal of a fifth EU freedom – the ‘freedom of knowledge’ – emerged from working towards a unified European research area, expanding upon the four basic freedoms of the Single Market. This additional freedom is not yet guaranteed and so this task should be taken up anew. Here, I support this goal by explicating the meaning of the ‘fifth freedom’, then justifying its importance via four arguments concerning scientific progress, freedom of research, the economic and technological progress of societies, and the democratic ideals of the European project. To end the main article, I then argue that this freedom should be viewed as a right. In the supplementary material, I present problems that arise in the administration, funding, and organization of science within the EU to demonstrate that the fifth freedom is still a long way from being a reality. The evidence includes cases of national rules and practices that reduce or block the mobility of EU researchers and weaken collaboration and progress. For each type of problem, I present specific policy proposals for advancing the fifth freedom goal. The proposals are directed at academic administrations, funding agencies, and political actors at the regional, national, and European levels.