Kolozs County was an administrative county (comitatus) of the Kingdom of Hungary, of the Eastern Hungarian Kingdom and of the Principality of Transylvania.

The rivers Crișul Repede and Someșul Mic flowed through the county.

In 1876, when the administrative structure of Transylvania was changed, the territory of Kolozs was modified and some villages of Doboka County (which was then disbanded) were annexed to it.

Following the Second Vienna Award, large part of it was retaken by Hungary in 1940 and the county was recreated; however, after World War II it became part of Romania again.

In the early 20th century, the subdivisions of Kolozs County were: