The Croatian Spring (Croatian: Hrvatsko proljeće), or Maspok, was a political conflict that took place from 1967 to 1971 in the Socialist Republic of Croatia, at the time part of the Socialist Federal Republic of Yugoslavia. As one of six republics comprising Yugoslavia at the time, Croatia was ruled by the League of Communists of Croatia (SKH), nominally independent from the League of Communists of Yugoslavia (SKJ), led by President Josip Broz Tito. The 1960s in Yugoslavia were marked by a series of reforms aimed at improving the economic situation in the country and increasingly politicised efforts by the leadership of the republics to protect the economic interests of their respective republics. As part of this, political conflict occurred in Croatia when reformers within the SKH, generally aligned with the Croatian cultural society Matica hrvatska, came into conflict with conservatives.

In the late 1960s, a variety of grievances were aired through Matica hrvatska, which were adopted in the early 1970s by a reformist faction of the SKH led by Savka Dabčević-Kučar and Miko Tripalo. The complaints initially concerned economic nationalism. The reformists wished to reduce transfers of hard currency to the federal government by companies based in Croatia. They later included political demands for increased autonomy and opposition to real or perceived overrepresentation of the Serbs of Croatia in the security services, politics, and in other fields within Croatia. A particular point of contention was the question of whether the Croatian language was distinct from Serbo-Croatian.

The Croatian Spring increased the popularity of figures from Croatia's past, such as the 19th century Croat politician and senior Austrian military officer, Josip Jelačić, and the assassinated leader of the Croatian Peasant Party, Stjepan Radić, as well as an increase in patriotic songs, works of art, and other expressions of Croatian culture. Plans were made for increased representation of Croatia-related materials in the school curriculum, measures to address the overrepresentation of Serbs in key positions in Croatia and to amend the Constitution of Croatia to emphasise the nature of the republic as the national state of Croats. There were also demands for increased powers for the constituent republics at the expense of Yugoslavia's federal government. These issues increased tensions between Croats and the Serbs of Croatia, as well as between the reformist and conservative factions of the SKH.

While other republics, the SKJ, and Tito himself were not initially involved in the internal Croatian struggle, the increasing prominence of Croatian nationalism led Tito and the SKJ to intervene. Similar to reformers in other Yugoslav republics, the SKH leadership was compelled to resign. Nevertheless, their reforms were left intact and most demands of the ousted leadership were later adopted, ushering in a form of federalism that contributed to the subsequent breakup of Yugoslavia.

In the early 1960s, the Federal People's Republic of Yugoslavia was a federation according to its constitution (comprising the people's republics of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, and Slovenia), but de facto operated as a centralised state. The Yugoslav economy was in recession, prompting economic reforms, which were hastily implemented and proved ineffective. By 1962, the country's economic difficulties worsened, prompting debate on the foundations of the economic system. In March 1962, President Josip Broz Tito convened the extended central committee of the country's ruling party, the League of Communists of Yugoslavia (SKJ), to discuss the role of the SKJ and the relationship between the central government and the constituent republics. The meeting exposed a clash between Serbs, openly supported by a Serb deputy prime minister Aleksandar Ranković, and Slovene members of the body, particularly Miha Marinko and Sergej Kraigher, cautiously supported by Slovene deputy prime minister Edvard Kardelj. The Slovene delegation advocated for devolving power and authority to the constituent republics. The Serb delegation sought to preserve the central government's monopoly on decision-making and the distribution of tax revenue to less-developed republics. As it was less developed than PR Slovenia and PR Croatia, PR Serbia would have benefited from such an arrangement. In 1963, a new constitution was adopted, granting additional powers to the republics, and the 8th Congress of the SKJ expanded the powers of the SKJ branches the following year.

Further economic reforms were adopted in 1964 and 1965, transferring considerable powers from the federation to the republics and individual companies. Some of the reform measures exacerbated conflict between the banks, insurers, and foreign trade organisations owned by the Yugoslav government versus those owned by the constituent republics, a conflict that became increasingly political and nationalist. Competing alliances were established. Ranković gained the support of Bosnia and Herzegovina and Montenegro, in addition to Serbia. Slovenia was supported by Croatia, based on the belief of Vladimir Bakarić—the Secretary of the Central Committee of the League of Communists of Croatia (SKH)—that decentralisation would benefit others in Yugoslavia. Bakarić persuaded Krste Crvenkovski, the head of the League of Communists of Macedonia (SKM), to support the Slovene–Croatian reformist bloc, which managed to enact substantial legislation curbing federal powers in favour of the republics. The conflict was framed as a contest between Serbia's interests against those of Slovenia and Croatia.

In Croatia, positions adopted by Ranković's allies in the League of Communists of Serbia (SKS) and the League of Communists of Montenegro (SKCG) were interpreted as hegemonistic, which in turn increased the appeal of Croatian nationalism. By the mid-1960s, the United States consul in Zagreb, Helene Batjer, estimated that about half of SKH members and 80 percent of the population of Croatia held nationalist views.

By early 1966, it was clear that the reforms had not produced the desired results. The SKJ blamed the Serbian leadership for resistance to the reforms. In early 1966, Kardelj persuaded Tito to remove Ranković from the SKJ Central Committee and dismiss him as vice president of Yugoslavia. Ranković was accused of plotting to seize power, disregarding the decisions of the eighth congress of the SKJ (December 1964), abuse of the State Security Administration directly or through allies, and illegally wire-tapping the SKJ leadership, including Tito himself. Tito saw Ranković's removal as an opportunity to implement greater decentralisation. In devolving power to constituent units of the federation, Tito assumed the role of sole arbiter in inter-republican disputes.