To determine the optimal evaluation method for the degradation of poly (butylene succinate) copolymer (PBSc) under seawater conditions, biodegradability under composting conditions was determined according to ISO 14855-2, the most general biodegradation test method for composting conditions, and the results were compared with those obtained under seawater conditions. Under composting conditions, after 45 days, the biodegradation of PBS-c reached 57% and the weight average molecular weight of PBS-c decreased from the initial 153000 g/mol to 44000 g/mol. Mean-while, under seawater conditions, the weight average molecular weight of PBS-c decreased to 94000 g/mol after 34 weeks. Although no significant changes in the chemical and crystal structure of PBS-c were observed after 34 weeks in seawater, the surface of the sample became rougher, as observed using scanning electron microscopy. The significantly slower biodegradation occurring under seawater conditions than under composting conditions can be attributed to the lower concentration of microorganisms in seawater at the lower test temperature. Therefore, a PBS-c degradation test was performed under accelerated conditions in seawater. Under accelerated conditions, the weight average molecular weight of PBS-c deceased to 29000 g/mol and substantial cracking and damage occurred on the surface of the PBS-c sample as a result of degradation.