The Great Mosque of Gaza, also known as the Great Omari Mosque, is a mosque in a ruinous state, located in Gaza City, in the State of Palestine. Prior to its 2023 partial destruction, it was the largest and oldest mosque in the Gaza Strip.

Believed to stand on the site of an ancient Philistine temple, the site was used by the Byzantines to erect a church in the 5th century. After the Rashidun conquest in the 7th century, it was transformed into a mosque. The Great Mosque's minaret was toppled in an earthquake in 1033. In 1149, the Crusaders built a large church. It was mostly destroyed by the Ayyubids in 1187, and then rebuilt as a mosque by the Mamluks in the early 13th century.

It was destroyed by the Mongols in 1260, then soon restored. It was destroyed by an earthquake at the end of the century. The Great Mosque was restored again by the Ottomans roughly 300 years later. Severely damaged after British bombardment during World War I, the mosque was restored in 1925 by the Supreme Muslim Council. In 2023, during the Gaza war and Gaza genocide, it was destroyed by the Israeli bombing of the Gaza Strip, leaving most of the structure collapsed and the minaret partially destroyed.

The Great Mosque is situated in the Daraj Quarter of the Old City in Downtown Gaza at the eastern end of Omar Mukhtar Street, southeast of Palestine Square. Gaza's Gold Market is located adjacent to it on the south side. To the northeast is the Katib al-Wilaya Mosque. To the east, on Wehda Street, is a girls' school.

According to tradition, the mosque stands on the site of the Philistine temple dedicated to Dagon—the god of fertility—which Samson toppled in the Book of Judges. Later, a temple dedicated to Marnas—god of rain and grain—was erected. Local legend today claims that Samson is buried under the present mosque.

A Christian basilica was built on the site in the 5th century CE, either during the reign of Eastern Roman Empress Aelia Eudocia, or Emperor Marcianus.^{[citation needed]} In either event, the basilica was finished and appeared on the 6th-century Madaba Map of the Holy Land.

The Byzantine church was transformed into a mosque in the 7th century by Omar ibn al-Khattab's generals, after the conquest of Roman Palaestina by the Rashidun Caliphate. The mosque is still alternatively named "al-Omari", in honour of Omar ibn al-Khattab who was caliph during the Muslim conquest of Palestine. In 985, during Abbasid rule, Arab geographer al-Muqaddasi wrote that the Great Mosque was a "beautiful mosque." On 5 December 1033, an earthquake caused the pinnacle of the mosque's minaret to collapse.

In 1149, the Crusaders, who had conquered Gaza in 1100, built a large church atop the ruins of the earlier Byzantine church upon a decree by Baldwin III of Jerusalem. However, in William of Tyre's descriptions of grand Crusader churches, it is not mentioned. Of the Great Mosque's three aisles today, it is believed that portions of two of them had formed part of the Crusader church.