Ethnic groups in Burundi include the three main indigenous groups of Hutu, Tutsi and Twa that have largely been emphasized in the study of the country's history due to their role in shaping it through conflict and consolidation. Burundi's ethnic make-up is similar to that of neighboring Rwanda. Additionally, recent immigration has also contributed to Burundi's ethnic diversity. Throughout the country's history, the relation between the ethnic groups has varied, largely depending on internal political, economic and social factors and also external factors such as colonialism. The pre-colonial era, despite having divisions between the three groups, saw greater ethnic cohesion and fluidity dependent on socioeconomic factors. During the colonial period under German and then Belgian rule, ethnic groups in Burundi experienced greater stratifications and solidification through biological arguments separating the groups and indirect colonial rule that increased group tensions. The post-independence Burundi has experienced recurring inter-ethnic violence especially in the political arena that has, in turn, spilled over to society at large leading to many casualties throughout the decades. The Arusha Agreement served to end the decades-long ethnic tensions, and the Burundian government has stated commitment to creating ethnic cohesion in the country since, yet recent waves of violence and controversies under the Pierre Nkurunziza leadership have worried some experts of potential resurfacing of ethnic violence. Given the changing nature of ethnicity and ethnic relations in the country, many scholars have approached the topic theoretically to come up with primordial, constructivist and mixed arguments or explanations on ethnicity in Burundi.

Indigenous population

Native Burundians belong to one of the three major ethnic groups in Burundi: the Hutu, Tutsi, and Twa peoples. The historical origins of ethnic differentiation between Hutu and Tutsi are disputed, however members of both groups consider themselves distinct. The relations between the indigenous ethnic groups in Burundi have greatly varied depending on the historical context or era. Generally, the distinctions have always existed, yet at certain periods they have produced greater violence than others. The ethnic dynamics between Hutus and Tutsis have especially shaped Burundian history and politics and have become a major object of study by scholars. Today, the Hutu are the largest ethnic group in Burundi, representing approximately 85 percent of Burundians. Under Colonial rule and then Burundi's Tutsi-dominated postcolonial government between 1965 and 2001, the Hutu population was marginalized and subordinated to the Tutsi elites. Since the end of the Burundian Civil War (1993–2005), however, Hutus have been dominant in political life. The Tutsi represent approximately 14 percent of the national population. Historically, the Tutsi dominated political institutions in Burundi, including the nation's monarchy, then their favoritism by colonial governments, and even after independence, the subsequent dictatorial regimes of Michel Micombero (1966–76), Jean-Baptiste Bagaza (1976–87), and Pierre Buyoya (1987–93) were dominated by Tutsi elites which often discriminated against the Hutu majority. The Twa make up around 1 percent of Burundi's population. As an ethnic group, they have links to the pygmy peoples of the Democratic Republic of the Congo. Nowadays, they often work as unskilled laborers. The Twa have historically been economically and politically marginalized by the other two ethnic groups. They are often disproportionately poor and suffer from legal discrimination.

Pre-colonial era While the exact nature and origins of the ethnic groups during the pre-colonial era Burundi is debated, many scholars have reached a consensus that ethnic divisions of Hutu, Tutsi and Twa groups did exist. Studies indicate that the Twa group was the first to settle in the area known as modern-day Burundi as artisans or hunter-gatherers. The presence of the largest group, the Hutus, came later around 1000 c.e. as a predominantly farmer group. The Tutsis arrived last and occupied positions such as cattle herders or pastoralists. The inter-group dynamic in pre-colonial Burundi was not one rooted in deep antagonisms. Divisions did exist, however, as the groups were solidified and distinct especially via labor. While socio-economic roles played a major role in distinguishing one group from the other, the role of biological features is less known. There did seem to be notable divisions between what were referred to as generally light-skinned, tall Tutsis and dark-skinned shorter Hutus, but the role and magnitude of such characteristics is unclear. Tutsis were seen as a wealthier and powerful group given their labor role and warfare abilities, while the Hutus and Twas were a less wealthy majority together. The emphasis on labor and wealth, however, did allow for fluidity within the groups as acquiring wealth or switching tasks could enable transitions. Hutus who acquired cattle, for example, could essentially become Tutsis and vice versa. This transition was not  always simple since there have been instances of ceremonies that confirm the conversion, indicating that wealth was not the only marker separating the groups, but one had to become the other formally. Descent was also emphasized as one was born into the categorizations, despite being able to change later. The overall emphasis on labor and its link to status has often led to the association of pre-colonial Burundian ethnicity to the idea of classes.

Consolidation under monarchy Given their wealth and warfare abilities, the shortage of land availability and conflict over it led to the Tutsi domination of Burundi in the 16th century through the establishment of a hierarchical monarchy ruled by divine kings or mwami, the first of which was Ntare I. The kings were Tutsi (at least in theory, though some would contend that they were more properly described as Ganwa); below them existed a separate class of princes, Ganwa, that were themselves potential heirs to the throne. This group usually distinguished themselves from the Hutu, Tutsi, and Twa masses. Many regions within Burundi were further controlled by Tutsi elites who were in turn loyal to the king. While this sort of hierarchy established a client-patron relation that especially revolved around royalty to Tutsi elites, there existed, both within the elites and especially in the masses, inter-mixing of groups. It was common for Hutus and Tutsis to intermarry on the level of the masses, and many Hutus to become Tutsi or of elite status via acquiring wealth. Even within the Tutsis there existed internal divisions between those who were regarded as “high”, or Tutsi-Banyaruguru, usually those belonging to the princely class, and those who were deemed as “low”, or Tutsi-Hima, who were often subordinate to wealthier Hutus. It could be said that the idea of ethnicity besides being more fluid, was also heterogeneous within the categories themselves. Despite the monarchy seemingly enhancing social hierarchies, many believed that its function also was in the interest of society at large as order alongside wealth was generated.

Colonial era

German rule After the Berlin Conference in 1884-5, Burundi came under German colonial sphere and then in 1890, it formally became a part of German East Africa. Given an inter-dynastic  struggle between the two groups of the Bezi and the Batare, Germany was able to exploit the monarchical conflict and access power by siding with the Bezi in promise of a formal recognition of German rule in Burundi.  Once in charge, Germany resorted to indirect rule given limitations of ability to rule directly and via settlement, and reached its objectives through the hierarchies in place. Out of fear of potential growth in the power of the monarch, German colonial rule empowered many of the chiefs in place which further enhanced the status of many Tutsi elites that had existed before. This also solidified the patron-client relations already subjugating many Hutus. Combined with the need to efficiently run Burundi, Germans began to note distinctions between the three groups of Hutus, Tutsis and Twas and searched for justifications of Tutsi favoritism. Given the preexisting divides and discourses of Social Darwinism, pseudo-scientific beliefs and studies began to surface, indicating that the Tutsis were descendants of people from Ethiopia, ancient Egypt, and Asia Minor, suggesting a closer connection to Europeans themselves. This was further advanced by generalizing biological distinctions between the groups and viewing the Tutsis as having more Eurocentric features. The amalgamation of such beliefs yielded to an anthropological hypothesis called the Hamitic hypothesis or “Hamitic myth”. The hypothesis argues that Tutsis were connected to the biblical Ham, and thus closer to the Christian world of Europeans themselves. This implied a genetic connection with East African peoples, although later scientific inquiries have refuted such claims. Additionally, a money-based economy was introduced to Burundi that served to change the previous nature of cattle as wealth, enabling a socio-economic mobility and further solidifying the ethnic categorizations.

Belgian rule The system of indirect colonial rule transferred to Belgium after Germany’s loss in WW1 and was reaffirmed through the Treaty of Versailles. The Belgians largely continued the German precedent of Tutsi favoritism and Hamitic beliefs and enhanced it through the creation of further divisions. Tutsis were granted the opportunity of education by Belgian Catholic priests, leading to many conversions to Catholicism and ideas of elitism due to the obtainment of formal education. Through the indirect oversight, Tutsis were able to impose forced, extractive labor on the majority Hutus, compensate Hutu lands, and impose taxes and mandatory cash crop collection. Such abuse by especially Tutsi elites, created a strong association between colonial rule and the Tutsi ethnic group in the eyes of Hutus and Twas. In the 1930s, Belgians introduced administrative centralization that enhanced Tutsi ranks in the administrative political, social and economic elites, especially the ganwa-Tutsi, at the expense of any Hutu presence in such positions of power. This was further overseen through introduction of identity cards indicating the ethnicity of individuals that hindered any flexibility between ethnic lines that occurred before. Furthermore, such cards helped determine who could be educated and the treatment of individuals according to their ethnicity. In a way, the groups were also separated by being ruled with different ‘customary laws’ dependent on ethnicity.