This article explores a tension in deliberative democratic theory. The tension consists in that deliberative opinion formation ideally aims to reach consensus, while a consensus, once established, will likely impede the conditions for further rational public discourse. Hence, over time, deliberative democracy might risk undermining itself. While the tension is demonstrable in theory, we also suggest three cognitive and socio-psychological mechanisms by which consensus might hamper the rationality of public discourse: after an agreement, participants cease to develop new arguments, they tend to forget existing arguments and their fear of deviating from the social norm promotes conformism. Existing research has largely neglected to study how consensus in decision making affects future public deliberation. Our article thus serves three purposes: to elaborate the consensus paradox in deliberative democratic theory; to open up a research agenda for examining the paradox empirically; and to assess the theoretical implications of the paradox.