Gemer (Slovak: Gemer, Hungarian: Gömör, German: Gemer or Gömör, Latin: Gömörinum) was an administrative county (comitatus) of the Kingdom of Hungary. In the 19th century, and in the beginning of the 20th century, it was united with the Kishont region to form Gömör-Kishont county (in Slovak: Gemer-Malohont). Its territory is now in southern Slovakia and northern Hungary. Today Gömör is only an informal designation of the corresponding territory.
Gemer county was situated in Slovenské rudohorie approximately between the present-day Slovak-Hungarian border, the towns Poltár and Rožňava and the Low Tatras (Nízke Tatry). The river Sajó flowed through the county. Its area was 4,289 km² around 1910.
The capital of Gemer/Gömör was the Gemer Castle/Gömör, and from the early 18th century Plešivec/Pelsőc. The capital after the addition of Malohont/Kishont was Rimavská Sobota/Rimaszombat.
Gömör/Gemer is one of the oldest counties of the Kingdom of Hungary, and was already mentioned in the 11th century. The territory approximately between the towns Tisovec and Rimavská Sobota, called Kishont/Malohont, was added from 1786 until 1790, and again in 1802. The county was called Gömör-Kishont/Gemer-Malohont since. She was also ruled by Ottomans between 1553-1686 as part of Nograd and Holok sanjaks in Buda and Uyvar eyalets.