The square is an excellent example of the architectural stratification that has developed over the centuries with the succession of various pontiffs.
The square's shape dates back to the 19th century, to a design by architect Giuseppe Valadier, who, commissioned by various popes, gave the square its current elliptical shape, with an Egyptian obelisk and ornamental neoclassical fountains at its center. The square encompasses within its perimeter the ancient Porta Flaminia, known as Porta del Popolo, built at the same time as the Aurelian Walls in the 3rd century as the northern entrance to the city, where the Via Flaminia began.

Next to Porta del Popolo is the ancient Basilica of Santa Maria del Popolo, erected in the 11th century at the behest of Pope Paschal II. During his pontificate, Pope Alexander VII commissioned Gian Lorenzo Bernini to restore the church. Between 1655 and 1661, Bernini gave the church a distinct Baroque feel and enriched it with numerous paintings and sculptures by great artists of the time. The basilica houses masterpieces such as the Conversion of Saint Paul and the Crucifixion of Saint Peter by Caravaggio, and the Assumption by Annibale Carracci in the Cerasi Chapel; the Chigi Chapel was designed and decorated by Raphael; and the Chapel of the Nativity features a fresco cycle by Pinturicchio. There are also sculptures by Andrea Bregno and Gian Lorenzo Bernini himself, including the magnificent organ supported by two bronze angels.

On the opposite side of the square from Porta del Popolo are two twin churches designed in 1662 by architect Carlo Rainaldi on behalf of Pope Alexander VII. The two churches are located at the ends of three straight lines that branch off southward and channeled traffic from Rome's main entrance to the major basilicas:

• Via di Ripetta toward Ponte Sant'Angelo and St. Peter's;
• Via del Corso reached Piazza Venezia and continued toward San Giovanni in Laterano;
• Via del Babuino, which, through Piazza di Spagna, ascended toward Santa Maria Maggiore.

The basilicas of Santa Maria in Montesanto (1675) and its twin, Santa Maria dei Miracoli (1678), were designed by architect Carlo Rainaldi and completed by Gian Lorenzo Bernini, lending the square a symmetry typical of the Baroque style. However, the two churches present differences precisely to adapt to the different spaces between the streets: Rainaldi, to maintain this perspective symmetry from the center of the square toward the central axis of Via del Corso, designed the dome of Santa Maria dei Miracoli to be perfectly circular, while the dome of Santa Maria in Montesanto is elliptical. In the 18th century, a quadrangular bell tower was added to the church of Santa Maria dei Miracoli, mirroring that of Santa Maria in Montesanto, precisely to emphasize the centrality of Via del Corso.

At the behest of Pope Sixtus V, a 23-meter-tall Egyptian obelisk, previously placed in the Circus Maximus by Emperor Constantine, was moved to the center of the square. The obelisk dates back to 1300 BC, built during the reign of Ramesses II, and was brought to Rome by Augustus to celebrate the conquest of Egypt. At the base of the obelisk, Valadier created the Fountain of the Lions: at the four corners of the obelisk are four white marble lions in Egyptian style, from whose jaws a jet of water flows.

At the center of the square's two semicircles, two other monumental fountains were built in 1822-23 by Giovanni Ceccarini based on a design by Valadier. On the Pincio side is the Fountain of the Goddess Roma, consisting of a large sculptural group with a statue of the Goddess Roma armed with a spear, flanked on either side by statues of the Tiber and Aniene rivers and the She-Wolf suckling the twins Romulus and Remus. On the opposite side is the Fountain of Neptune, consisting of a statue of Neptune armed with a trident in his right hand, flanked by tritons and dolphins.