The interior (and dome) has an elliptical plan, with a dodecagonal cupola.

"Rainaldi here had both the job of town planner and church architect. He designed this impressive piazza with the two churches Santa Maria dei Miracoli and Santa Maria di Montesanto opposite the Porta del Popolo. Three main streets radiate from the Piazza to the heart of Rome, two of each of the three flanking one of the apparently twin churches. The large domes, one oval and one round, appear identical from the Piazza. This adjustment in plan was necessitated by differing sites. The classical temple front was influenced by Bernini."