The national flag of Guinea-Bissau was adopted on the day Guinea-Bissau proclaimed its independence from Portugal in 24 September 1973. It is almost identical to the flag of the African Party for the Independence of Guinea and Cape Verde, the country's dominant party and previously sole ruling party.

The black star symbolises the leadership of the PAIGC, with its colour representing the people of Africa. Yellow and green correspond to Guinea-Bissau's northern savannas and southern forests, respectively, and the two are also meant to represent agriculture. Little legislation has been made on the proper colours of the flag, though a piece of legislation from 1993 states, "Anyone who, publicly, by words, gestures or written disclosures, or by other means of communication with the public, outrages the Republic, the national flag or anthem, the weapons or emblems of Guinean sovereignty or fails to respect due to him, is punished up to three years in prison."

Like the flag of Cape Verde upon independence, the flag of Guinea-Bissau is based on that of the African Party for the Independence of Guinea and Cape Verde (PAIGC), which is the same but with the party's initials below the black star. The party adopted its flag in 1961, echoing the Pan-African colours used by the existing flags of neighbouring Ghana and Guinea, namely red, yellow, and green. The black star symbolises the leadership of the PAIGC, with its colour representing the people of Africa. Yellow and green correspond to Guinea-Bissau's northern savannas and southern forests, respectively, and broadly speaking the yellow is said to refer to agriculture and the fruits of labour, while green is said to represent the agriculture in the jungle. The yellow is also sometimes reported to represent the Sun or mineral wealth, green represents hope, and red represents bloodshed in the revolution. Wikipedia