Pan-Arabism (Arabic: الوحدة العربية, romanized: al-waḥda al-ʿarabiyyah, lit. 'Arab unity') is a pan-nationalist ideology that espouses the unification of all Arab people in a single nation-state, consisting of all Arab countries of West Asia and North Africa from the Atlantic Ocean to the Arabian Sea, which is referred to as the Arab world. It is closely connected to Arab nationalism, which asserts the view that the Arabs constitute a single nation. It originated in the late 19th century among the Arab regions of the Ottoman Empire, and its popularity reached its height during the peak of Nasserism and Ba'athism in the 1950s and 1960s. Advocates of pan-Arabism have often espoused Arab socialist principles and strongly opposed the political involvement of the Western world in the Arab world. It also sought to empower Arab states against outside forces by forming alliances such as the Arab League.

The origins of pan-Arabism are often attributed to the Nahda (Arab awakening or enlightenment) movement that flourished in the Arab regions of the Ottoman Empire in the late 19th century. A prominent figure was Jurji Zaydan (1861–1914), who played a key role in laying the intellectual foundation for Pan-Arabism. Zaydan had critical influence on acceptance of a modernized version of the Quranic Arabic language (Modern Standard Arabic) as the universal written and official language throughout the Arab world, instead of adoption of local dialects in the various countries. Zaydan wrote several articles during the early 20th century which emphasized that Arabic-speaking regions stretching from the Maghreb to the Persian Gulf constitute one people with a shared national consciousness and that this linguistic bond trumped religious, racial and specific territorial bonds, inspired in part by his status as a Levantine Christian émigré in 19th century Egypt. He also popularized through his historical novels a secular understanding of Arab history encompassing the pre-Islamic and Islamic periods into a shared history that all Arabs could claim as their own.

As a political project, pan-Arabism was first pressed by Sharif Hussein ibn Ali, the Sharif of Mecca, who sought independence for the Mashreq Arabs from the Ottoman Empire, and the establishment of a unified Arab state in the Mashreq. In 1915 and 1916, the Hussein-McMahon Correspondence resulted in an agreement between the United Kingdom and the Sharif that if the Mashreq Arabs revolted successfully against the Ottomans, the United Kingdom would support claims for Mashreq Arab independence. In 1916, however, the Sykes-Picot Agreement between the United Kingdom and France determined that parts of the Mashreq would be divided between those powers rather than forming part of an independent Arab state. When the Ottoman Empire surrendered in 1918, the United Kingdom refused to keep to the letter of its arrangements with Hussein, and the two nations assumed guardianship of Mesopotamia, Lebanon, Palestine and what became modern Syria. Ultimately, Hussein became King of only Hijaz, in the then less strategically valuable south, but lost his Caliphate throne when the kingdom was sacked by the Najdi Ikhwan forces of the Saudites and forcefully incorporated into the newly created Kingdom of Saudi Arabia.

A more formalized pan-Arab ideology than that of Hussein was first espoused in the 1930s, notably by Syrian thinkers such as Constantin Zureiq, Sati' al-Husri, Zaki al-Arsuzi, and Michel Aflaq. Aflaq and al-Arsuzi were key figures in the establishment of the Arab Ba’ath (Renaissance) Party, and the former was for long its chief ideologist, combining elements of Marxist thought with nationalism to a considerable extent reminiscent of nineteenth-century European romantic nationalism. It has been said that Arsuzi was fascinated with the Nazi ideology of "racial purity" and impacted Aflaq.

Abdullah I of Jordan dreamed of uniting Syria, Palestine, and Jordan under his leadership in what he would call Greater Syria. He unsuccessfully proposed a plan to this effect to the United Kingdom, which controlled Palestine at that time. The plan was not popular among the majority of Arabs and fostered distrust among the leaders of the other Middle Eastern countries against Abdullah. The distrust of Abdullah's expansionist aspirations was one of the principal reasons for the founding of the Arab League in 1945. Once Abdullah was assassinated by a Palestinian nationalist in 1951, the vision of Greater Syria was dropped from the Jordanian agenda.

Although pan-Arabism began at the time of World War I, Egypt (the most populous and arguably most important Arab country) was not interested in pan-Arabism prior to the 1950s. Thus, in the 1930s and 1940s, Egyptian nationalism – not pan-Arabism – was the dominant mode of expression of Egyptian political activists. James Jankowski wrote about Egypt at the time,

What is most significant is the absence of an Arab component in early Egyptian nationalism. The thrust of Egyptian political, economic, and cultural development throughout the nineteenth century worked against, rather than for, an 'Arab' orientation. ... This situation—that of divergent political trajectories for Egyptians and Arabs—if anything increased after 1900.

It was not until Gamal Abdel Nasser that Arab nationalism (in addition to Arab socialism) became a state policy and a means with which to define Egypt's position in the Middle East and the world, usually articulated vis-à-vis Zionism in the neighboring state of Israel.