The national flag of Norway (Bokmål: Norges flagg; Nynorsk: Noregs flagg) is red with a navy blue Scandinavian cross fimbriated in white that extends to the edges of the flag; the vertical part of the cross is shifted to the hoist side in the style of the Dannebrog, the flag of Denmark.

It is difficult to establish what the earliest flag of Norway looked like. During ancient times countries did not fly flags. Kings and other rulers flew flags, especially in battle. Saint Olav used a serpent within a white mark at the Battle of Nesjar. Prior to this the raven or dragon was used. Magnus the Good used the same mark as Saint Olav. Harald Hardrade used the raven banner. This flag was flown by various Viking chieftains and other Scandinavian rulers during the 9th, 10th, and 11th centuries AD. Inge used a red lion on gold. Sverre used an eagle in gold and red. The earliest known flag which could be described as a national flag of Norway is the one used today as the Royal Standard. Eirik Magnusson used a flag described as a golden lion with axe and crown on red from 1280 and this was since regularly the flag of the King of Norway and thereby also of Norway.

The flag is based on the coat of arms and was originally only a flag for the ruler of Norway (as it is today). It was later also used on ships and on fortresses until it was gradually phased out during the 17th and 18th centuries. Its earliest certain depiction is on the seal of duchess Ingebjørg in 1318. Around 1500 it became the custom for ships to fly the flag of their home country to identify their nationality. A red flag with the golden lion and silver halberd is depicted as the flag of Norway in a Dutch flag book from 1669 to 1670. At least as late as 1698 the lion banner was flown over Akershus Fortress. The "Norwegian lion" was placed in the colours of all the Norwegian regiments in 1641. In 1748 a decree stated that the Dannebrog should be the only legal merchant flag.

From about the 16th century until 1814, Norway used the same flag as Denmark, as it was in union with that country. In 1814 independent Norway adopted the Danish flag with the Norwegian lion in the canton or the upper square at the hoist. This flag was in use as state and war flag until 1815 and as merchant flag until 1821. Later in 1814 Norway was united with Sweden, and on 7 March 1815 a common war flag for both states was introduced by royal order in council, the Swedish flag with a white cross on a red background in the square canton. The same design in a rectangular flag was introduced as an alternate merchant flag in 1818, for use in distant waters, i.e. south of Cape Finisterre in Spain.

The current flag of Norway was designed in 1821 by Fredrik Meltzer, a member of the parliament (Storting). It was adopted by both chambers of the Storting on 11 May and 16 May, respectively. However, the king refused to sign the flag law, but approved the design for civilian use by royal order in council on 13 July 1821. The constitution of 1814 explicitly stated that the war flag was to be a union flag, hence the common flag (Swedish with a canton signifying Norway) was used by the armies and navies of both states until 1844.

Until 1838 the Norwegian flag was only used in Northern waters, i.e. the waters north of Cape Finisterre, as Norway had no treaty with the Barbary pirates of North Africa and had to fly the Swedish or union flag for protection. In 1844 a union mark combining Norwegian and Swedish colours was placed at the hoist of both countries' flags. The badge was jokingly or derogatorily called Sildesalaten ("the herring salad") because of its jumble of colours and resemblance to a popular dish on the breakfast tables of both countries. Initially, the union flag was popular in Norway, since it clearly denoted the equal status of the two united states. As the union with Sweden became less popular, the Norwegian parliament abolished the union mark from the national (merchant) and state flags in 1898. Although the law was not approved by the King, it became effective since it had been passed by three consecutive Stortings. The "pure" flag was first flown in 1899, but the union mark had to be kept in the war flag. After the dissolution of the union, it was removed from the navy flag as well on 9 June 1905. Sweden kept it in all flags until 1 November 1905.