The Republic of Serbian Krajina or Serb Republic of Krajina (Serbian: Република Српска Крајина / Republika Srpska Krajina, pronounced [rɛpǔblika sr̩̂pskaː krâjina]; abbr. РСК / RSK), known as the Serbian Krajina (Српска Крајина / Srpska Krajina) or simply Krajina (Крајина), was an unrecognized geopolitical entity and a self-proclaimed Serb quasi-state, a territory within the newly independent Republic of Croatia (formerly part of Socialist Yugoslavia), which it defied, and which was active during the Croatian War of Independence (1991–95). It was not recognized internationally. The name Krajina ("Frontier") was adopted from the historical Military Frontier of the Habsburg monarchy (Austria-Hungary), which had a substantial Serb population and existed up to the late 19th century. The RSK government waged a war for ethnic Serb independence from Croatia and unification with the Federal Republic of Yugoslavia and Republika Srpska (in Bosnia and Herzegovina).

The government of Krajina had de facto control over central parts of the territory while control of the outskirts changed with the successes and failures of its military activities. The territory was legally protected by the United Nations Protection Force (UNPROFOR). Its main portion was overrun by Croatian forces in 1995 and the Republic of Serbian Krajina was ultimately disbanded as a result. A rump remained in eastern Slavonia under UNTAES administration until its peaceful reintegration into Croatia in 1998 under the Erdut Agreement.

The name Krajina (meaning "frontier") stemmed from the Military Frontier which the Habsburg monarchy carved out of parts of the crown lands of Croatia and Slavonia between 1553 and 1578 with a view to defending itself against the expansion of the Ottoman Empire. The population was mainly Croats, Serbs and Vlachs who immigrated from nearby parts of the Ottoman Empire (Ottoman Bosnia and Serbia) into the region and helped bolster and replenish the population as well as the garrisoned troops in the fight against the Ottomans. The Austrians controlled the Frontier from military headquarters in Vienna and did not make it a crown land, though it had some special rights in order to encourage settlement in an otherwise deserted, war-ravaged territory. The abolition of the military rule took place between 1869 and 1871. In order to attract Serbs to become part of Croatia, on 11 May 1867, the Sabor solemnly declared that "the Triune Kingdom recognizes the Serbs living in it as a nation identical and equal with the Croatian nation". Subsequently, the Military Frontier was incorporated into Habsburg Croatia on 1 August 1881 when the Ban of Croatia Ladislav Pejačević took over from the Zagreb General Command.

Following the end of World War I in 1918, the regions formerly forming part of the Military Frontier came under the control of the Kingdom of Yugoslavia, where they formed part of the Sava Banovina, along with most of the old Croatia-Slavonia. Between the two World Wars, the Serbs of the Croatian and Slavonian Krajina, as well as those of the Bosnian Krajina and of other regions west of Serbia, organized a notable political party, the Independent Democratic Party under Svetozar Pribićević. In the new state there existed much tension between the Croats and Serbs over differing political visions, with the campaign for Croatian autonomy culminating in the assassination of a Croatian leader, Stjepan Radić, in the parliament, and repression by the Serb-dominated security structures.

Between 1939 and 1941, in an attempt to resolve the Croat-Serb political and social antagonism in first Yugoslavia, the Kingdom established an autonomous Banovina of Croatia incorporating (amongst other territories) much of the former Military Frontier as well as parts of Bosnia and Herzegovina. In 1941, the Axis powers invaded Yugoslavia and in the aftermath the Independent State of Croatia (which included the whole of today's Bosnia and Herzegovina and parts of Serbia (Eastern Syrmia) as well) was declared. The Germans installed the Ustaše (who had allegedly plotted the assassination of the Serbian King Alexander I of Yugoslavia in 1934) as rulers of the new country; the Ustaše authorities promptly pursued a genocidal policy of persecution of Serbs, Jews and Croats (from opposition groups), leading to the deaths of over 300,000. During this period, individual Croats coalesced around the ruling authorities or around the communist anti-fascist Partisans. Serbs from around the Knin area tended to join the Chetniks, whilst Serbs from the Banovina and Slavonia regions tended to join the Partisans. Various Chetnik groups also committed atrocities against Croats across many areas of Lika and parts of northern Dalmatia.

At the end of World War II in 1945, the communist-dominated Partisans prevailed and the Krajina region became part of the People's Republic of Croatia until 7 April 1963, when the federal republic changed its name to the Socialist Republic of Croatia. Josip Broz Tito suppressed the autonomous political organizations of the region (along with other movements such as the Croatian Spring); however, the Yugoslav constitutions of 1965 and 1974 did give substantial rights to national minorities - including to the Serbs in SR Croatia.

The Serbian "Krajina" entity to emerge upon Croatia's declaration of independence in 1991 would include three kinds of territories:

Large sections of the historical Military Frontier lay outside of the Republic of Serbian Krajina and contained a largely Croat population - these including much of Lika, the area centered around the city of Bjelovar, central and south-eastern Slavonia.