The national flag of Portugal (Portuguese: Bandeira de Portugal) is a rectangular bicolour with a field divided into green on the hoist, and red on the fly. The lesser version of the national coat of arms of Portugal (armillary sphere and Portuguese shield) is centered over the colour boundary at equal distance from the upper and lower edges. Its presentation was done on 1 December 1910, after the downfall of the constitutional monarchy on 5 October 1910. However it was only on 30 June 1911, that the official decree approving this flag as the official flag was published. This new national flag of the First Portuguese Republic, was selected by a special commission whose members included Columbano Bordalo Pinheiro, João Chagas and Abel Botelho.

The conjugation of the new field colours, especially the use of green, was not traditional in the Portuguese national flag's composition and represented a radical republican-inspired change that broke the bond with the former monarchical flag. Since a failed republican insurrection on 31 January 1891, red and green had been established as the colours of the Portuguese Republican Party and its associated movements, whose political prominence kept growing until it reached a culmination period following the Republican revolution of 5 October 1910. In the ensuing decades, these colours were popularly propagandised as representing the hope of the nation (green) and the blood of those who died defending it (red), as a means to endow them with a more patriotic and dignified, therefore less political, sentiment.

The current flag design represents a dramatic change in the evolution of the Portuguese standard, which had always been closely associated with the royal arms, blue and white. Since the country's foundation, the national flag developed from the blue cross-on-white armorial square banner of King Afonso I to the liberal monarchy's arms over a blue-and-white rectangle. In between, major changes associated with determinant political events contributed to its evolution into the current design.