The world's largest mosque is either the Masjid al-Haram, located in Mecca, Saudi Arabia, or the Imam Reza Shrine, which stands in Mashhad, Iran. The al-Haram mosque is by far the largest mosque in the world by its total capacity, while the Reza Shrine covers the greatest area.
The Masjid al-Haram mosque was first constructed in the seventh century and has been expanded multiple times since. To accommodate the huge flood of pilgrims who visit every year during Ramadan, the mosque has a capacity of 4 million people. The Imam Reza Shrine can only contain 1.5 million people at once, but at 598,657 square meters, it is almost half again as large as the al-Haram site.