Háromszék county shared borders with Romania and the Hungarian counties Csík, Udvarhely, Nagy-Küküllő, and Brassó. The river Olt flowed through the county. The Carpathian Mountains formed its southern and eastern border. Its area was 3,889 km2 (1,502 sq mi) around 1910.
Háromszék means "three seats". Háromszék County was a combination of three seats of the Székelys: Kézdiszék, Orbaiszék, and Sepsiszék (plus some villages of the former Felső-Fehér County). The county was formed in 1876, when the administrative structure of Transylvania was changed.
Afterward, it became part of Romania again; its territory lies mainly in the present Romanian county of Covasna, with a small part in the south being part of Brașov County.