The Leaning Tower of Pisa (Italian: Torre pendente di Pisa) or simply the Tower of Pisa (Torre di Pisa) is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa, known worldwide for its unintended tilt. It is situated behind the Cathedral and is the third oldest structure in Pisa's Cathedral Square (Piazza del Duomo) after the Cathedral and the Baptistry. The tower's tilt began during construction, caused by an inadequate foundation on ground too soft on one side to properly support the structure's weight. The tilt increased in the decades before the structure was completed, and gradually increased until the structure was stabilized (and the tilt partially corrected) by efforts in the late 20th and early 21st centuries.

The height of the tower is 55.86 metres (183.27 feet) from the ground on the low side and 56.67 metres (185.93 feet) on the high side. The width of the walls at the base is 2.44 m (8 ft 0.06 in). Its weight is estimated at 14,500 metric tons (16,000 short tons). The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees. This means that the top of the tower is displaced horizontally 3.9 metres (12 ft 10 in) from the centre.

Construction of the tower occurred in three stages across 199 years. Work on the ground floor of the white marble campanile began on August 14, 1173, during a period of military success and prosperity. This ground floor is a blind arcade articulated by engaged columns with classical Corinthian capitals. The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for almost a century, because the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca, and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled. In 1198 clocks were temporarily installed on the third floor of the unfinished construction.

In 1272 construction resumed under Giovanni di Simone, architect of the Camposanto. In an effort to compensate for the tilt, the engineers built upper floors with one side taller than the other. Because of this, the tower is actually curved. Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria. The seventh floor was completed in 1319. It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the bell-chamber with the Romanesque style of the tower. The bell-chamber was finally added in 1372. There are seven bells, one for each note of the musical major scale. The largest one was installed in 1655. After a phase (1990–2001) of structural strengthening, the tower is currently undergoing gradual surface restoration, in order to repair visible damage, mostly corrosion and blackening. These are particularly pronounced due to the tower's age and its exposure to wind and rain.