Provided is a binder composition for a lithium ion secondary battery positive electrode that can suppress expansion of a positive electrode. The disclosed binder composition for a lithium ion secondary battery positive electrode includes a copolymer containing a nitrile group-containing monomer unit and a conjugated diene monomer unit and a solvent. A degree of swelling in electrolysis solution of the copolymer is at least 200 mass % and no greater than 700 mass % and a storage elastic modulus of a binder coating obtained through film formation of the binder composition is at least 1×104 and no greater than 1×109 Pa.