This paper proposes a definition of populism from the distinction made by Karl R. Popper between the concepts of open and closed society, considering globalization as a new formulation of the open society and populism as a new reaction to the closed society. Popper conceives the open society as a tolerant and pluralist one, whereas the closed society is the opposite and tends to take place as a response to the first one. For Popper, the closed society has taken various forms throughout history, including both the platonic aristocratic reaction to the Athenian democracy and the Nazi totalitarianism reaction to Weimar democracy. It is here argued that it is possible to define populism based on the three elements that characterize closed societies according to Popper: historicism, collectivism and anti-rationalism. Populism can be understood as a political ideology opposed to liberal democracy, which arises as a response to some consequences of the development of contemporary liberal democratic societies, particularly as a reaction to the phenomenon of globalization. Taking globalization as the contemporary open society, populism can thus be defined as the current closed society.