Ecosystem service values (ESVs) of bays and their response to sea reclamation are of great practical importance for forming bay eco-compensation policy and extension of blue economic space. Based on land use information of bays collected during the period of 1990–2015, the spatiotemporal evolution of ESVs of 12 main bays in East China Sea and their response to sea reclamation activities over the past 25 years were quantitatively analyzed. The analysis results indicate that ESVs of bays in East China Sea showed a continuous downward trend and the whole ecosystem was continuously degraded, in which the degradation degree of ESV in the southern bays was higher than that in the northern bays. Spatial zoning of ESVs of bays in East China Sea was remarkable, showing a continuous downward trend from low-value to high-value zone. Spatial variation of ESVs of each bay was also significant, expanding from a city and from inland to the coast, which suggests that human activities, mainly reclamation, have become main agents for ESV evolution of bays in East China Sea. ESVs of bays have a significant response to sea reclamation, manifested as a significant negative correlation between ESV and reclamation intensity. The correlation in the southern bays was stronger than that in the northern bays, which was caused by different effects of sea reclamation modes on ESV evolution of muddy and bedrock bays. A negative effect of sea reclamation activities on bay ecosystem was hysteretic. Therefore, an attention should be paid to dynamic monitoring and early warning of development status in offshore areas, ecosystem-level reclamation control policy, and coastal wetland reserves planning. Moreover, the spatial coupling mechanism study between bay ecosystem service demand and its service supply capacity should be strengthened to realize systematic regulation of bay ecological security pattern.