The neoclassical exterior expresses the traditional Russian-Byzantine formula of a Greek-cross ground plan with a large central dome and four subsidiary domes. It is similar to Andrea Palladio's Villa La Rotonda, with a full dome on a high drum substituted for the Villa's low central saucer dome. The design of the cathedral in general and the dome in particular later influenced the design of the United States Capitol dome, Wisconsin State Capitol in Madison, Wisconsin, and the Lutheran Cathedral in Helsinki.

The exterior is faced with gray and pink stone, and features a total of 112 red granite columns with Corinthian capitals, each hewn and erected as a single block: 48 at ground level, 24 on the rotunda of the uppermost dome, 8 on each of four side domes, and 2 framing each of four windows. The rotunda is encircled by a walkway accessible to tourists. 24 statues stand on the roof, and another 24 on top of the rotunda.

Relief is a sculptural technique where the sculpted elements remain attached to a solid background of the same material. The term relief is from the Latin verb relevo, to raise. To create a sculpture in relief is to give the impression that the sculpted material has been raised above the background plane. What is actually performed when a relief is cut in from a flat surface of stone (relief sculpture) or wood (relief carving) is a lowering of the field, leaving the unsculpted parts seemingly raised. The technique involves considerable chiselling away of the background, which is a time-consuming exercise. On the other hand, a relief saves forming the rear of a subject, and is less fragile and more securely fixed than a sculpture in the round, especially one of a standing figure where the ankles are a potential weak point, especially in stone. In other materials such as metal, clay, plaster stucco, ceramics or papier-mâché the form can be just added to or raised up from the background, and monumental bronze reliefs are made by casting.