The Portuguese First Republic (1910–1926) was marked by significant instability at the most diverse levels. With a special focus on the financial dimension of this period, the objective of this paper is to test the sustainability of the Portuguese fiscal policy, also referred to as the sustainability of public finances itself. The methodology involves testing the stationarity of public debt and budget balance and also the cointegration between state revenue and expenditure. The results obtained shows that the state’s intertemporal budgetary constraint was violated during the First Republic regime, which denotes unsustainability. This conclusion is justified by the existence of a non-stationary budget balance and the absence of cointegration between state revenue and expenditure. These results are manifestly different from those that have already been obtained for other Portuguese regimes, namely for the Estado Novo (1933–1974) and democracy (1974–present), where sustainability existed. This paper is yet another demonstration of how important it is to maintain control of state’s accounts. We hope that this paper can be useful to stimulate new research on Portuguese public finances and also on the important issue of fiscal policy sustainability.