Ploiești ( , , ), formerly spelled Ploești, is a city and county seat in Prahova County, Romania. Part of the historical region of Muntenia, it is located north of Bucharest.
The area of Ploiești is around , and it borders the Blejoi commune in the north, Bărcănești and Brazi communes in the south, Târgșoru Vechi commune in the west, and Bucov and Berceni communes in the east. According to the 2021 census, Ploiești is the tenth most populous city in the country with a population of 180,540.
The city grew beginning with the 17th century on an estate bought by ruler Michael the Brave from the local landlords, gradually replacing nearby Wallachian fairs of Târgșor, Gherghița, and Bucov. Its development was accelerated by heavy industrialisation during the mid-19th century, with the world's first large-scale petroleum refinery being opened between 1856 and 1857. Following massive exploitation of the oil deposits in the area, Ploiești earned the nickname of "the Capital of Black Gold". In the present, a significant part of its economic activity is still based on oil processing, the city having three large refineries and other industries related to this branch.
Ploiești is also an important transport hub, linking the capital with the regions of Transylvania and Moldavia. The city has direct access to Prahova Valley, one of the most important alpine tourism areas in Romania.
History
Though likely settled much earlier, Ploiești first appeared in documents in the 16th century during the reign of Michael the Brave, the Prince of Wallachia (). It flourished as a center for trade and handicraft-manufacturing in the 17th and 18th centuries. The road connecting Ploiești to Brașov was opened in 1864, and the railway arrived in 1882. Many schools and hospitals date from this period.
In the mid-19th century, the region of Ploiești became one of the world's leading oil-extraction and -refinery sites. The Mehedințeanu brothers opened the world's first large refinery in Ploiești in 1856–1857. History also remembers the city as the site of the self-styled Republic of Ploiești, a short-lived 1870 revolt against the Romanian monarchy.
During World War I, Ploiești's oil production made it a target when the Central Powers invaded Romania in 1916, a British Army operation commanded by Colonel John Griffiths destroying production and sabotaging much of the infrastructure of the industry.
thumb|left|Columbia Aquila refinery burning after the raid of B-24 Liberator bombers, [[Operation Tidal Wave, in 1943]]
Although badly damaged after the November 1940 earthquake, the city functioned as a significant source of oil for Nazi Germany during much of World War II. The Allies made Ploiești a target of the oil campaign of World War II and bombed it repeatedly,
such as during the HALPRO (Halverson Project, June 1942) and Operation Tidal Wave (1 August 1943) at a great loss, without producing any significant delay in operation or production. Soviet Red Army troops captured Ploiești on 24 August 1944.
Following the war, the new Communist régime of Romania nationalised the oil industry, which had largely been privately owned, and made massive investments in the oil- and petroleum-industry in a bid to modernise the country and to repair the war damage.