Pärnu (Estonian pronunciation: [ˈpærˑnu]) is the fourth-largest city in Estonia. Situated in southwest Estonia, Pärnu is located 128 kilometres (80 mi) south of the Estonian capital, Tallinn, and 176 kilometres (109 mi) west of Estonia's second-largest city, Tartu. The city sits off the coast of Pärnu Bay, an inlet of the Gulf of Riga, which is a part of the Baltic Sea. In the city, the Pärnu River drains into the Gulf of Riga.

Pärnu is a popular summer holiday resort town among Estonians with many hotels, restaurants and large beaches. The city is served by Pärnu Airport.

Pärnu or Old Pärnu (Latin: Perona, German: Pernau, Alt-Pernau, Estonian: Vana-Pärnu), which was founded by the bishop of Ösel–Wiek c. 1251, suffered heavily under pressure of the concurrent town, and was finally destroyed c. 1600. Another town, Embeke (later German: Neu-Pernau, Estonian: Uus-Pärnu) was founded by the Livonian Order, who began building an Ordensburg nearby in 1265. The latter town, then known by the German name of Pernau, was a member of the Hanseatic League and an important ice-free harbor for Livonia. The Polish–Lithuanian Commonwealth took control of town between 1560 and 1617; the Poles and Lithuanians fought the Swedes nearby in 1609. Sweden took control of the town during the 16th-century Livonian War as part of Swedish Livonia, although it was not formally ceded by Poland-Lithuania until the 1660 Treaty of Oliva. Sweden then lost Livonia to the Russian Empire in the 1710 Capitulation of Estonia and Livonia and the 1721 Treaty of Nystad, following the Great Northern War; the city also bore the name Пернов, Pernov—an "adapted" German name—among the Russians. It belonged to the Imperial Russian Governorate of Livonia until 1917, when it was transferred to the short-lived Autonomous Governorate of Estonia. The city is occasionally referred to as Pyarnu, an incorrect reverse-transliteration from the Russian Пярну.

The town became part of independent Estonia in 1918 following World War I and the Estonian War of Independence.

The city was occupied by the Soviet Red Army along with the rest of Estonia in 1940 during World War II, and its German population fled the town. It was briefly occupied by Germany from 1941 until 1944 before it was liberated by the Soviet Union during its counteroffensives. Pärnu then continued as being part of the Estonian Soviet Socialist Republic from 1944 to 1991, when Estonia restored its independence.

During the Great Northern War, the Academia Gustavo-Carolina in Dorpat (cf. University of Tartu) was relocated from Dorpat to Pärnu from 1699 to 1710. The university has still maintained a branch campus in Pärnu to this day (1,000 students in the 2004/2005 school year).

There are seven districts in Pärnu: Ülejõe, Rääma, Vana-Pärnu, Kesklinn, Rannarajoon, Eeslinn and Raeküla.

Pärnu lies within the temperate humid continental climate zone.