Pyhäjärvi (until 1968, in official contexts, often Pyhäjärvi Ol (Oulu County) as a difference from other municipalities of the same name, in 1993–1995 Pyhäsalmi) is a Finnish city located in the southern part of the province of Northern Ostrobothnia. The city has a population of 5,090 people and an area of ​​1,459.46 km², of which 148.66 km² are water bodies. The population density is 3.88 inhabitants / km². The neighboring municipalities of Pyhäjärvi are Haapajärvi, Kiuruvesi, Kärsämäki, Pielavesi, Pihtipudas and Pyhäntä. At the beginning of 2017, Pyhäjärvi became the second smallest municipality in Finland in terms of population, after Kaskinen, after Juankoski joined Kuopio.

Until 1968, the municipality of Pyhäjärvi was officially named Pyhäjärvi Ol, in 1969–1992 Pyhäjärvi, in 1993–1995 Pyhäsalmi and again from 1996 named Pyhäjärvi. Other municipalities in Finland called Pyhäjärvi have been Pyhäjärvi Ul, which now belongs to Karkkila, and Pyhäjärvi Vpl, which was located in the ceded Karelia. Lake Pyhäjärvi became a city in 1993.

Along the regional road 560 leading to Pielavesi, there is a large boulder Rillankivi, which unites the borders of three municipalities - Pyhäjärvi, Pielavesi and Pihtiputa - and three provinces - Northern Ostrobothnia, Northern Savonia and Central Finland. Until 1997, the stone was a common landmark of the counties of Oulu, Central Finland and Kuopio. Pyhäjärvi has the deepest (1.4 km) and most modern metal mine in Europe, owned by the Canadian company First Quantum Minerals, from which ore containing zinc and copper is mined. In late summer, the Full Moon Dances Festival will be held in Pyhäjärvi, which is the most significant contemporary dance event in Finland.