Ostrobothnia County (Swedish: Österbottens län, Finnish: Pohjanmaan lääni) was a county of the Swedish Empire from 1634 to 1775.
The county was split in 1755 to County of Vasa (Swedish: Vasa län, Finnish: Vaasan lääni) and County of Uleåborg (Swedish: Uleåborg län, Finnish: Oulun lääni).
Provinces of Finland 1634: 1: Turku and Pori, 14: Nyland and Tavastehus, 18: Ostrobothnia
, 20: Viborg and Nyslott, 21: Kexholm
Provinces of Finland 1776: 1: Turku and Pori, 4: Vaasa
, 10: Oulu
, 14: Nyland and Tavastehus, 15: Kymmenegård, 16: Savolax and Karelia