The Imam Reza shrine is a complex of religious buildings in Mashhad, Iran, and is the largest mosque in the world by dimension and the second-largest by capacity. The shrine contains the tomb of Ali al-Ridha, the eighth Shia Imam, who died in 818 AD.

The mosque has undergone numerous renovations and expansions over the centuries, with the most recent renovation completed in 1994. The complex covers an area of over 267,000 square meters and includes a number of buildings, including several mosques, a library, a museum, and a hospital.

The central building of the complex is the Imam Reza shrine, which consists of four courtyards and seven prayer halls, and is covered by a massive golden dome that stands at over 35 meters tall. The shrine is considered one of the holiest sites in Shia Islam, and is visited by millions of pilgrims from around the world every year.

The Imam Reza shrine is also home to one of the most important libraries of Islamic manuscripts in the world, with over 20,000 handwritten copies of the Quran and other Islamic texts. Additionally, the complex includes a museum that displays artifacts and artworks related to the history of Shia Islam.

The Imam Reza shrine is not only a religious site, but also a cultural and social center for the city of Mashhad. The complex is surrounded by a bustling market, and hosts a variety of events and festivals throughout the year.