Ida-Viru County (Estonian: Ida-Viru maakond or Ida-Virumaa; Russian: Ида-Вируский уезд, romanized: Ida-Viruskij uezd) is one of the 15 counties of Estonia. It is the most northeastern part of the country. The county contains large deposits of oil shale – the main mineral mined in Estonia. Oil shale is used in the production of shale oil and in thermal power plants. The capital of the county is the town of Jõhvi which is administratively united with the Jõhvi Parish; nevertheless, Narva is the largest town in the county in terms of population and at the same time the third-largest city in Estonia after Tallinn and Tartu.
In January 2019, Ida-Viru County had a population of 136,240 – constituting 10.3% of the total population in Estonia. It borders Lääne-Viru County in the west, Jõgeva County in the southwest and Russia (Leningrad Oblast) in the east. It is the only county in Estonia where Russians constitute the majority of population (73.1% in 2010), the second highest being Harju (28%).