The national flag of the Portuguese Republic, often referred to as the Portuguese flag, consists of a rectangular bicolour with a field divided into green on the hoist, and red on the fly. The version without laurels of the country’s national coat of arms stands in the middle of the Portuguese armillary sphere and shield, centered over the colour boundary at equal distance. The flag was announced in 1910, following the 5 October 1910 revolution, inspired by the colours of the Republican Party and the design of radical conspiratorial society Carbonária.

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 for 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 color, 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, green represented the hope of the nation and the colour red represented the blood of those who died defending it, this happened to endow them with a more patriotic and dignified, therefore less political, sentiment.

The sphere and shield in the middle of the current flag are an integral part of the design, which has historically been centred on the royal arms, usually over fields of blue and white. Since the country's foundation, the standard developed from the blue cross-on-white armorial square banner of King Afonso I, through progressively more complex designs, which did incorporate green and red, 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.

The decree that legally created the republican flag was approved by the Constituent Assembly and published in government journal no. 141 (Portuguese: diário do Governo) on 19 June 1911. On 30 June, this decree had its regulations officially published in government diary no. 150. Nevertheless, the new flag had been first presented in 1910, on the national holiday of the Restoration of Independence (celebrating December 1, 1640), which was rededicated as the Day of the Flag, although this new designation did not erase the previous one.

The flag's length is equal to 1+1⁄2 times its width, which translates into an aspect ratio of 2:3. The background is vertically divided into two colours: dark green on the hoist side, and scarlet red on the fly. The colour division is made in a way that green spans 2⁄5 of the length and the remaining 3⁄5 is filled by red (ratio 2:3). The lesser version of the national coat of arms (without the laurel wreaths)—a white-rimmed national shield on top of a black-highlighted yellow armillary sphere—is positioned over the border between both colours.

The armillary sphere has a diameter equal to 1⁄2 height and is equidistant from the upper and lower edges of the flag. The sphere, drawn in perspective, possesses six edge-embossed arcs, four of which are great circles and two are small circles. The great circles represent the ecliptic (wider oblique arc), the equator, and two meridians. These last three are positioned so that the intersections between each two arcs make a right angle; one meridian lies on the flag's plane, while the other is perpendicular to it. The small circles consist of two parallels (the tropics), each tangent to one of the ecliptic-meridian intersections.

Vertically centered over the sphere is the national shield, a white-rimmed curved bottom red shield charged with a white inescutcheon. Its height and width are equal to 7⁄10 and 6⁄10 of the sphere's diameter, respectively. The shield is positioned in a way that its limits intersect the sphere:

A curious aspect of the official design is the absence of a segment of the Tropic of Capricorn, between the national shield and the ecliptic arc.