The Piazza dei Miracoli (Italian: [ˈpjattsa dei miˈraːkoli]; 'Square of Miracles'), formally known as Piazza del Duomo ('Cathedral Square'), is a walled 8.87-hectare (21.9-acre) compound in central Pisa, Tuscany, Italy, recognized as an important center of European medieval art and one of the finest architectural complexes in the world.

Partly paved and partly grassed, the Piazza dei Miracoli is also the site of the Ospedale Nuovo di Santo Spirito ('New Hospital of the Holy Spirit'), which now houses the Sinopias Museum (Museo delle Sinopie) and the Cathedral Museum (Museo dell'Opera del Duomo).

Also in the façade is found the tomb of Buscheto (on the left side) and an inscription about the foundation of the cathedral and the victorious battle against the Saracens.

At the east end of the exterior, high on a column rising from the gable, is a modern replica of the Pisa Griffin, the largest known Islamic metal sculpture, the original of which was placed there probably in the 11th or 12th century, and is now in the Cathedral Museum.

The impressive mosaic of Christ in Majesty, in the apse, flanked by the Virgin and Saint John the Evangelist, survived the fire.

Galileo is believed to have formulated his theory about the movement of a pendulum by watching the swinging of the incense lamp (not the present one) hanging from the ceiling of the nave.

The elaborately carved pulpit (1302–1310), which also survived the fire, was executed by Giovanni Pisano, and is a masterpiece of medieval sculpture.

The pulpit is supported by plain columns (two of which are mounted on lion's sculptures) on one side and by caryatids and a telamon on the other: the latter represent Saint Michael, the Evangelists, the four cardinal virtues flanking the church, and a bold, naturalistic depiction of a naked Hercules.

That tomb, originally in the apse just behind the main altar, was disassembled and moved many times over the centuries for political reasons.

The exact moment is determined by a ray of sun that, through a window on the left side, falls on an egg-shaped marble, just above the pulpit by Giovanni Pisano; this occurs at noon.

Some relics brought back during the Crusades can also be found in the cathedral: alleged remains of three saints (Abibo, Gamaliel, and Nicodemus), and a vase that is said to be one of the jars of Cana.

It was built in Romanesque style by an architect known as Diotisalvi ("God Save You"), who worked also in the church of the Holy Sepulchre in the city.

It is the largest baptistery in Italy, with a circumference measuring 107.25 m. Taking into account the statue of St. John the Baptist (attributed to Turino di Sano) atop the dome, it is even a few centimetres taller than the Leaning Tower.

The portal, facing the façade of the cathedral, is flanked by two classical columns, while the inner jambs are executed in the Byzantine style.

The scenes on the pulpit, and especially the classical form of the naked Hercules, show at best Nicola Pisano's abilities as the most important precursor of Italian renaissance sculpture by reinstating antique representations.

The last of the three major buildings on the piazza to be built, construction of the bell tower began in 1173 and took place in three stages over the course of 177 years, with the bell-chamber only added in 1372.

On 27 July 1944, incendiary bombs dropped by Allied aircraft set the roof of the building on fire and covered them in molten lead, all but destroying them.

The Ospedale Nuovo di Santo Spirito (New Hospital of Holy Spirit) is located on the south area of the square.

In 1562, during the time when the Medici dominated the city, the hospital was restructured according to Florentine renaissance canons; all the doors and windows were modified with new rectangular ones encased in grey sandstone.

Since 1976, the middle part of the building contains the Sinopias Museum, where original drawings of the Campo Santo frescoes are kept.

The Palazzo dell'Opera (Opera in the sense of "works" - these were the workshops of the complex) is at the south east corner of the square.

Originally these houses belonged to the workmen of the cathedral complex: the tailor, the gardener, the bell ringers, etc., until the 19th century when the administration offices of the Opera della Primaziale were moved in.