Hagia Sophia was built by the Byzantine Emperor Justinian I in the 6th century. Justinian I ruled the Byzantine Empire from 527 to 565 and is known for his ambitious building projects, including the construction of Hagia Sophia.

The cathedral was built to be the grandest and most impressive in the Byzantine Empire and was intended to symbolise the empire's wealth, power, and religious devotion. 

Justinian I also wanted the cathedral to be a centre of spiritual and intellectual activity, and it played a significant role in the development of the Eastern Orthodox Church.

Construction of Hagia Sophia began in 532 and was completed in just six years, an impressive feat considering the scale and complexity of the project. 

The building was designed by two architects, Anthemius of Tralles and Isidorus of Miletus, using innovative architectural techniques ahead of their time. The cathedral was dedicated to the Holy Wisdom of God (Hagia Sophia in Greek) and became one of the most important religious and cultural centres in the Byzantine Empire.