Al-Omari Grand Mosque (Arabic: المسجد العمري الكبير) is a mosque in Beirut Central District, Lebanon. It was originally a Roman temple, and subsequently a Roman church, before Beirut was conquered by Mamluk Egypt and it was converted into a mosque.

The Al-Omari Grand Mosque was originally a Roman temple to the god Jupiter. During the Byzantine era it was made into a Roman church. During the Christian crusades it modified into the Church of Saint John in the 12th century CE. It was then converted after Islamic conquest into the city's Grand Mosque by the Mamluks in 1291 after the crusades ended. Damaged during the Lebanese Civil War, the mosque's refurbishment was completed in 2004.

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The Al-Omari Grand Mosque was originally a Roman temple to the god Jupiter.

During the Byzantine era it was made into a Roman church. It was converted into the Church of Saint John during the 12th century during the Crusades. Similar Romanesque churches with triple apses were built in Tyre and Tartus, using recuperated material such as Roman columns and capitals.

In 1291, the Mamluks captured Beirut, and under Islamic conquest it was converted into a mosque. It was renamed Al-Omari Mosque after the second caliph, and became known as "Jami' Al-Kabir" (the Great Mosque). Its Mamluk-style entrance and minarets were added in 1350, reflecting traces of the former church's Byzantine architecture.

Badly damaged during the Lebanese Civil War (1975–90), the mosque's refurbishment was completed in 2004.

• Romanesque style

• Al-Wali, Sheikh Mohammad Taha (1973) Tarikh al-masajid wal jawami' al-sharifa fi Bayrout, Dar al-Kotob, Beirut.
• Enlart, Camille (1904) "La Cathédrale Saint-Jean de Beyrouth" in: Société Nationale des Antiquaires de France: centenaire 1804-1904, Klincksieck, Paris: 121-133.
• Information provided by architect Youssef Haidar, Beirut.