The architectural appearance of St. Isaac's Square, which the south facade of the cathedral faces, had been forming for a long time, up to the beginning of the 20th century.

In 1839-1844 on the banks of the Moika River, opposite the cathedral, the building of the Mariinsky Palace was constructed according to the project of A. I. Stackenschneider. The center of the facade is highlighted by a portico with a massive attic and a protruding arcade of the main entrance. The flag of the Russian Federation flies over the building: The Legislative Assembly of St. Petersburg is located here.

The rectangular part adjacent to the Moika river was framed by two symmetrical buildings intended for the Ministry of State Property (architect N.E. Efimov) in 1844-1853. The facades of both buildings are decorated with Renaissance architecture motifs. Buildings compositionally balance each other. Now they host the N. I. Vavilov All-Russian Institute of Plant Genetic Resources.

In the central part of St. Isaac's Square in 1856-1859 a monument to the emperor Nicholas I was erected. The architectural solution of the monument belongs to O. Montferrand. The equestrian sculpture sculpted by P.K. Klodt has only two points of support. It is mounted on a high figured pedestal with allegorical sculptures and reliefs on historical themes.

The square garden between the monument and the cathedral was created in 1860.

House 11 on the square was designed by a prominent German architect P. Behrens in 1911-1912. The building was intended for the German embassy. Nowadays it houses the Office of the Ministry of Justice and the Main Technical Commission under the President of the Russian Federation for the North-West Federal District.

The Astoria Hotel, faced with gray granite (1911-1912, architect F. I. Lidval) is located on the opposite corner of the square. It was considered the best in Russia at a time. The hotel complex now includes a neighboring house - hotel Angleterre. It was in one of its rooms where the life of a famous Russian poet, S. A. Yesenin tragically ended on December 27, 1925.

House 9 is the oldest building on the square. This three-story mansion was built in the 1760s and hosted such guests as D. Didro, J. de Stael and A. Schlegel, when they visited Saint Petersburg.

House on the corner of St. Isaac's Square and Malaya Morskaya Street is associated with the name of F. M. Dostoevsky. The writer lived here in 1848-1849. In this house he wrote the "White Nights" novel.

House 7 on the corner of St. Isaac's Square and Pochtamtskaya Street belonged to the family of the Decembrist A. M. Bulatov at the beginning of the XIX century. In 1825, it was one of the centers of the Decembrist movement. The Decembrists K. F. Ryleev, A. A. Bestuzhev, I. I. Pushchin, P. G. Kakhovsky gathered here at the poet A. I. Odoevsky’s rented apartment