Saint Isaac's Cathedral is the largest Russian Orthodox cathedral in the city and was the tallest Eastern Orthodox church upon its completion (subsequently surpassed only by the Cathedral of Christ the Saviour). It is dedicated to Saint Isaac of Dalmatia, a patron saint of Peter the Great who had been born on the feast day of that saint.

The cathedral is a Late Neoclassical rendering of a Byzantine Greek-cross church.

The design of the cathedral in general and the dome in particular later influenced the design of the United States Capitol in Washington, D.C. and the Cathedral in Helsinki.