The first signs of the people presence in this region are since the Paleolithic era, and then the shepherds of free Dacian period, but the first inhabitants of the nowadays settlement may be considered 20 serfs from Petros , settled in Petroseni in 1640.

Petrosani official story begins in 1788, when the name of the village is recorded in the book “Journey from Potsdam to Constantinople,” by the Prussian officer Gotze. Only in 1818 the name of Petrosani is mentioned in official documents, attesting it in the census with 233 inhabitants, whose main occupation was shepherding.

After 1840, the region will experience a rapid development, after starting the surface mining of the coal discovered in the area that would become the largest coalfield in Romania and one of the most important in Europe.
The history of coal and its discovery in Petrosani is closely connected with the name of Hoffman brothers from Rusca Banatului, also with a nobleman named Maderspach Charles, successor of some large lands in the Jiu Valley, and with the “Society of mines and furnaces” from Braşov, which in 1850 started “the exploitation” in the south-western direction.

Intrigued by those findings, the state advised the mining director from Zlatna (K.K. Berghauptmannschaft) to do research into the Jiu Valley, so that in 1857, 16 fixed points are set to start the future exploitation.  State reserves, therefore, for exploitation in-house, the hills from Petrila, so the north-western part.
On the other hand, the Society from Brasov owned until 1865 a total of 77 parts of land, being financed by the consortium “Wiener Bankverein” from Vienna, and the state reserves for itself 71 such parts. Neither one nor the other started the exploitation, because in the absence of a railway, could not be profitable.

In 1865, following a plan developed by the engineer Freund, the work on the railways started and the new route, from Simeria to Petrosani, officially opens on August 18th 1870.

With that opening track work, it also started the effective exploitation of the coal. The society from Brasov started exploitation in 1867-1868, and on December 24th 1868, the State also started coal mining in-house. But in 1894, the society from Brasov put the mines up for sale and they were purchased by the Company Salgotarjan; in 1896, it took over the state mines too for a period of 12 years. In 1908, the state took back the mines, for exploitation in-house. There were 5418 miners in Petrosani then, who were producing 9,637,400 m3 of coal.

Thus, we can say that after 1848, the city development is closely linked to the development of the mining industry. In 1918, after the Great Union, Petrosani region is included in the new administrative-territorial organization of Hunedoara County. Until 1920, Petrosani was known as Petroseni (name kept even today by peasants and natives, so-called “momârlani”), but from February 14th, 1921 the settlement will become as we know it today, namely Petrosani.

On January 4th 1924, Petrosani becomes town and then, municipality in March 1st, 1968. From 1948, also, the town of Petrosani becomes university centre by establishing “the Coal Institute”, today, the University of Petrosani, that has to become one of the most prestigious Higher Education Institution – mining profile in Romania and Europe.

On the other hand, nowadays, on the administrative territory of Petrosani, there is Livezeni Mine (located on Lunca Street, no. 153), the headquarters of the Hunedoara Energy Complex (Timişoara Street, no. 2), but also a museum, the only one in our country with technical mining profile, Mining Museum (N. Balcescu Street, no. 2), and from 1996,  Petrosani hosts one of the most prestigious research institutions in Romania, National Institute for Research and Development in Mine Safety and Protection to Explosion (INSEMEX ) (General Vasile Milea Street, no. 32-34).

According to the census from 2011, Petrosani city population is 37,160 inhabitants, in declining from the previous census in 2002, when it had registered 45,195 inhabitants. Most inhabitants are Romanian (83.22%). The main minorities are the Hungarian (6.05%) and Roma (1.61%). For 8.55% of the population, the ethnicity is not known. From the religious point of view, most of the inhabitants are Orthodox (76.92%), but there are minorities of Catholics (6.66%), Protestants (2.74%) and Pentecostals (2.1%). For 8.64% of the population, the confession is not known.