St.Isaac's Cathedral is St.Petersburg's most magnificent church, built on a colossal scale, which makes it one of the largest domed churches in the world (333ft high). A church dedicated to St.Isaac of Dalmatia, whose saint's day coincided with Peter the Great's birthday.

A church was built to the design of the French architect Auguste de Montferrand in 1818-1848. Finely constructed of red granite and grey marble, the Cathedral is rectangular in plan, though with its four massive porticoes it looks like domed cruciform church. The gilded central dome is 85ft in diameter. The interior of the Cathedral is particularly rich. The walls are so lavishly decorated with many different kinds of marble, precious and semi-precious stones, as well as over 200 mosaics and pictures that the individual details almost loose their effect. Altogether more than 43 different minerals were used in the construction and decoration of the Cathedral, earning it the name of a "museum of Russian geology".

After the October Revolution the Cathedral ceased to be used for worship, and in 1928 it was decide to convert it into a museum, which opened to the public in 1931.