Ploiești (UKplɔɪˈɛʃti , USplɔːˈjɛʃt(i) , ploˈjeʃtj), 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 Michael the Brave from the local landlords, gradually taking the place of the nearby Wallachian fairs of Târgșor, Gherghița and Bucov. Its evolution was accelerated by heavy industrialisation, with the world's first industrial scale petroleum refinery being opened in 1856–1857. Following massive exploitation of the oil deposits in the area, Ploiești earned the nickname of "the Capital of Black Gold". In the present, 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 the Prahova Valley, one of the most important alpine tourism areas in Romania.
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 (1593-1601). 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.