Pan-Islamism (Arabic: الوحدة الإسلامية, romanized: al-Waḥdat al-Islāmiyya) is a political movement which advocates the unity of Muslims under one Islamic state, often a caliphate or an international organization with Islamic principles. Historically, after Ottomanism, which aimed at the unity of all Ottoman citizens, Pan-Islamism was promoted in the Ottoman Empire during the last quarter of the 19th century by Sultan Abdul Hamid II for the purpose of preventing secession movements of the Muslim peoples in the empire.

Pan-Islamism differentiates itself from pan-nationalistic ideologies, for example Pan-Arabism, by focusing on religion and not ethnicity and race. It sees the ummah (Muslim community) as the focus of allegiance and mobilization, including the Tawhid belief by the guidance of Quran and Sunnah's teachings.

The major leaders of the Pan-Islamist movement were the triad of Jamal al-Din al-Afghani (1839–1897), Muhammad Abduh (1849–1905) and Rashid Rida (1865–1935), who were active in anti-colonial efforts to confront European penetration of Muslim lands. They also sought to strengthen Islamic unity, which they believed to be the strongest force to mobilize Muslims against imperial domination. Following Ibn Saud's conquest of the Arabian Peninsula, pan-Islamism would be bolstered across the Islamic world. During the second half of the 20th century, pan-Islamists competed against left-wing nationalist ideologies in the Arab world such as Nasserism and Ba'athism. At the height of the Cold War in the 1960s and 1970s, Saudi Arabia and allied countries in the Muslim world led the Pan-Islamist struggle to fight the spread of communist ideology and curtail the rising Soviet influence in the world.

The Arabic term Ummah, which is found in the Quran and Islamic tradition, has historically been used to denote the Muslims as a whole, regardless of race, ethnicity, etc. This term has been used in a political sense by classical Islamic scholars e.g. such as al-Mawardi in Al-Ahkam al-Sultaniyyah, where he discusses the contract of Imamate of the Ummah, "prescribed to succeed Prophethood" in protection of the religion and of managing the affairs of the world. Al-Ghazali also talks about Ummah in a political sense e.g. in his work, "Fadiah al-Batinyah wa Fadail al-Mustazhariyah".

Fakhruddin al-Razi, who also talks about Ummah in a political sense, is quoted as saying the following:

The world is a garden, whose waterer is the dynasty, which is the authority. The guardian of this authority is the Shari'ah and Shari'ah is also the policy which preserves the kingdom; the kingdom is the city which the army brings into existence; the army is guaranteed by wealth; wealth is acquired by the subjects (Ummah) who are made servants via justice; justice is the axis of well being of the world.

— al-Razi in his Jami al-'Ulum

The ideology takes as its model the early years of Islam – the reign of Muhammad and the early caliphate – especially during Islamic golden age, as it is commonly held that during these years the Muslim world was strong, unified, and free from corruption.