The Modruš-Rijeka County was a historic administrative subdivision of the Kingdom of Croatia-Slavonia. Croatia-Slavonia was an autonomous kingdom within the Lands of the Crown of Saint Stephen (Transleithania), the Hungarian part of the dual Austro-Hungarian Empire. Its territory is now in western Croatia. Modruš is a small town near Ogulin; Rijeka is a large city on the Adriatic coast. However, Rijeka was not part of the Modruš-Rijeka County, but under the direct administration of Hungary. The capital of the county was Ogulin.

The counties of Croatia are the first-level administrative subdivisions of the Republic of Croatia. Since they were re-established in 1992, Croatia has been divided into 20 counties and the capital city of Zagreb, which has the authority and legal status of both a county and a city. As of 2015, the counties are subdivided into 128 cities and 428 municipalities. The divisions have changed over time since the medieval Croatian state. They reflected territorial losses and expansions; changes in the political status of Dalmatia, Dubrovnik and Istria; and political circumstances, including the personal union and subsequent development of relations between the Kingdom of Croatia-Slavonia and the Kingdom of Hungary.