The Piazza lies inside the northern gate in the Aurelian Walls, once the Porta Flaminia of ancient Rome, and now called Porta del Popolo. This was the starting point of the Via Flaminia, the road to Ariminum (modern Rimini ) and the most important route to the north. For centuries, the Piazza del Popolo was a place for public executions, the last of which took place in 1826.

In 1561, Pope Pio IV de' Medici commissioned Nanni de Baccio Bigio to replace the ruined medieval gate in the Aurelian wall with what became known as the Porta Flaminia. In 1572, Pope Gregory XIII commissioned Giacomo Della Porta to build a fountain which was later moved to Piazza Nicosia. In 1589, Pope Sisto V Peretti ordered Domenico Fontana to re-erect the Flaminio obelisk in the center of the piazza. In 1655 upon the visit of Queen Cristina of Sweden's the gate was renamed Porta del Popolo. In 1815, Giuseppe Valadier redesigned the piazza to give it it's oval symmetry by adding the walls. Valadier also added the central fountain and the four white Egyptian lions around the obelisk, with water falling like fans into round basins.

Landscaping with water features. The outdoor water fountains found in Rome's Piazza del Popolo is regarded as part of a grand architectural design incorporating the piazza's shape, dimensions, location and surroundings and not as a set of modular water features. This composition is the work of Giuseppe Valadier (1762 – 1839) the Italian architect, designer, urban planner, archeologist and chief exponent of Neoclassicism in Italy.

Valadier really did succeed in making the Piazza an elegant whole composed of elegant water feature designs. This is particularly true of the outdoor stone water fountains he created which are under-stated, innovative, classic, imposing yet graceful all at the same time. Behind the shell fountains, for example, you’ll see depictions of groups. There is Mars resplendent in armor and the river gods Tiber and Anio. The other group consists of Neptune and two Tritons.