Sectarianism in Lebanon refers to the formal and informal organization of Lebanese politics and society along religious lines. It has been formalized and legalized within state and non-state institutions and is inscribed in its constitution. Lebanon recognizes 18 different sects: 55% of the population is Muslim (27% Sunni, 27% Shia, small percentage of Alawites and Ismailis), 40.5% is Christian, the majority being Maronites Catholics and Greek Orthodox (with smaller groups including Greek Catholics, Armenian Orthodox, Armenian Catholics, Syriac Orthodox, Syriac Catholics, Assyrians, Chaldean Catholics, Copts, Protestants), while 4.52% is Druze. The foundations of sectarianism in Lebanon dates back to the mid-19th century during Ottoman rule. It was subsequently reinforced with the creation of the Republic of Lebanon in 1920, during the French Mandate era and the 1926 constitution, and in the National Pact of 1943. In 1990, with the Taif Agreement, the constitution was revised but did not structurally change aspects relating to political sectarianism. The dynamic nature of sectarianism in Lebanon has prompted some historians and authors to refer to it as "the sectarian state par excellence" because it is a mixture of religious communities and their myriad sub-divisions, with a constitutional and political order to match.

Despite the religious nature of sectarian affiliations, sectarianism in Lebanon is commonly considered to be a political project, as it not only relies on, but also reproduces, complex and unstable relations between religious and sectarian affiliation, on the one hand, and politics, violence, conflict, and co-existence, on the other. Sectarianism can therefore in Lebanon best be seen as a religious division that dictates: how people treat each other in daily life, determines their personal legal status and how the political parties are divided. Through the sectarian discourse, religion becomes the defining characteristic of the public and political subject, following a logic that has been established by (religious) authorities.

Some scholars and journalists define sectarianism as fixed pre-existing communal categories in society, and use it to explain political, cultural, or religious conflicts between groups. Other scholars conceive of sectarianism as a set of social practices where daily life is organised on the basis of communal norms and rules that individuals strategically use and transcend. This definition highlights the understanding of sectarianism as being co-constituted by people’s practices and agency, rather than rooted in fixed and incompatible communal boundaries.

Historians have argued that the origins of sectarianism lay at the "intersection of nineteenth-century European colonialism and Ottoman modernization." The traditional order of Lebanese society during Ottoman domination was not shaped along sectarian lines. Ottoman Lebanese society could be described as divided between an elite community that controlled religious and secular knowledge and common villagers that constituted the bulk of the society. Elite membership was thus determined by rank rather than religious affiliation, and relied on familial power constructed through a network of family alliances that often cut across religious lines. However, between 1840 and 1860, this social order began to be questioned and transformed, and religion entered the political sphere. The end of Ibrahim Pasha's occupation in 1840 created a power vacuum that various actors tried to exploit. Some of these actors relied on a religious base for their claims: the Maronite Church, for instance, called for the restoration of a Christian emirate in the region, while Maronite peasants asked for equality in the name of the Edict of Gulhane which in 1839 had guaranteed the rights of Ottoman citizens regardless of their religion. Moreover, the establishment of a new order in Mount Lebanon became the arena for the clash between European and Ottoman interests. While the Ottoman Empire attempted centralization through the Tanzimat reforms, European powers favoured local autonomies on religious bases. This European practice altered the meaning of religion in the multi-confessional society because it "emphasized sectarian identity as the only viable marker of political reform and the only authentic basis for political claims."

In the decades that followed, a colonial strategy and technique to assert control and perpetuate power used by the French during their mandate rule of Lebanon was divide and rule, and during this era sectarianism was institutionalized through the codification of personal status laws. The establishment of the Ja'fari court in 1926, facilitated by the French as a "quasi-colonial institution", provided Shi'a Muslims with sectarian rights through the institutionalization of Shi’a Islam, and hence gave rise to political Shi’ism. The "variation in the institutionalization of social welfare across different sectarian communities forged and exacerbated social disparities". Additionally, with the standardization, codification and bureaucratization of Shi’a Islam, a Shi’i collective identity began to form and the Shi’i community started to "practice" sectarianism. "The French colonial state contributed to rendering the Shi‘i community in Jabal ‘Amil and Beirut more visible, more empowered, but also more sectarian, in ways that it had never quite been before." This fundamental transformation led by the French created a new political reality that paved the way for the "mobilization" and "radicalization" of the Shi’a community during the Lebanese civil war.

Subsequently, the formation of the modern Lebanese nation-state in 1920 and its official independence in 1943 was founded on politicized sectarianism as the only form of governance and presumed egalitarianism between individual citizens and recognized sectarian groups." Hence, "in many ways sectarian identification converged with personal status, or madhhab, whereby the state recognized one official personal status per sect." Eighteen sects were officially recognized by the Mandate authorities. Regarding the Shi’i sect, this group was not officially recognized under the Ottoman Empire, while "the expansion of sectarian rights under the French mandate laid the foundation for Shi’i citizens in the postcolonial state, which entailed the formation of Ja’fari shari’a courts."

Meanwhile, when it came to the school system during the French mandate Lebanon's sectarianism was further allowed to establish itself. In 1920 the French founded the "Service de l’instruction publique" (SIP) whose task it was to oversee the education system in the French mandates. This French government body allowed different religious groups in Lebanon to establish their own religious schools with their own curriculum. Although the SIP did take control over the curriculum of the public schools in Lebanon. The non-religious private schools however would also be granted the right to establish their own curriculum in 1924, however this did come with the clause that a limited amount of French had to be taught in the school. Although the different religious groups were able to establish their own schools, students did not necessarily select a school solely based on their sectarian affiliation. While sect affiliation was one factor in school choice, other considerations—such as social class, political ideology, cultural alignment, and geographical proximity—also played a significant role. Consequently, although community affiliations influenced educational structures, the boundaries within the education system were often more fluid than those within the state apparatus.

However, so far, the main emphasis of the majority of research has been on the top-down sectarianism, whereby policies and institutionalization have formed the main aspect of research. What is often forgotten is the bottom-up approach towards sectarianism. Scholars like Linda Sayed and Nadya Sbaiti have studied sectarianism from this often neglected approach and have come to the conclusion that citizens themselves have had a more flexible understanding of this segregation of groups in their society. After all, sectarianism became institutionalised in Lebanon only upon it becoming a French mandate. Its population was thus used to the non-sectarian institutions of the late Ottoman Empire and had yet to be accustomed to the newly established sectarian institutions. For instance, Linda Sayed shows that Shi’i citizens used the court-system, created by the French rulers, in a rather pragmatic way than in a rigid sectarian sense and Nadya Sbaiti shows that people used the school-system in the same pragmatic fashion, creating communities of knowledge. In other words, people’s choices regarding the education of their children was rather based on guaranteeing at least a basic education "in the hope of some incremental if not dramatic betterment. Their decisions often centered around this goal and evinced less concern with the allegedly ‘unique’ confessional-communal characteristics reflected by a given school."