The First Portuguese Republic spans a complex 16-year period in the history of Portugal, between the end of the period of constitutional monarchy marked by the 5 October 1910 revolution and the 28 May 1926 coup d'état. The latter movement instituted a military dictatorship known as Ditadura Nacional (national dictatorship) that would be followed by the corporatist Estado Novo (new state) regime of António de Oliveira Salazar.

The sixteen years of the First Republic saw nine presidents and 44 ministries, and were altogether more of a transition between the Kingdom of Portugal and the Estado Novo than they were a coherent period of governance.