Provided are a copolymer containing: a structural unit (A) derived from a polymerizable monomer (a) having a silicon atom-containing group represented by formula (1) [wherein R¹, R², and R³each independently represent a hydrocarbon group having a carbon number of 1 to 6]; and a structural unit (B) derived from a polymerizable monomer (b) copolymerizable with the polymerizable monomer (a) with the content of the structural unit (A) being from 40 mass% to 80 mass% in 100 mass% of the copolymer, the content of the structural unit (B) being from 20 mass% to 60 mass% in 100 mass% of the copolymer, and an acid value derived from free carboxy group being from 0.1 KOHmg/g to 60 KOHmg/g.; and an antifouling coating composition containing the copolymer.