Marx devoted little attention to the functions of the State (and, particularly, to the role of public debt) in the capitalist system, and in more recent times, little attention has been devoted to this issue on the part of Marxist scholars. Starting from a reconstruction of Marx’s view on this issue, this article aims to analyze the effects of the expansion of public debt on capital reproduction in Marx’s thought and to derive a criterion of public debt sustainability consistent with Marx’s view. by contrast to the dominant interpretations, there is no single answer, for Marx, to the relation between public expenditure, capital reproduction, and income distribution. Finally, it has been shown that a criterion of public debt sustainability can be derived from Marx and that this criterion is political in essence, implying that the expansion of public debt cannot generate a condition where wages are persistently settled below their subsistence level.