An antifouling coating composition contains a silyl ester (co)polymer and medetomidine and is used to prevent the fouling of substrates by aquatic organisms and which can form antifouling coating films exhibiting outstanding antifouling properties over a long period and also has good storage stability. The antifouling coating composition includes a silyl ester (co)polymer (A) and medetomidine (B), the silyl ester (co)polymer (A) including structural units derived from a monomer (a) represented by the general formula (I): R1—CH═C(CH3)—COO—(SiR2R3O)n—SiR4R5R6, and structural units derived from an unsaturated monomer (b) copolymerizable with the monomer (a).