Abstract Coastal wetlands are increasingly impacted by tropical cyclones and storm surges. Seawalls and levees built to reduce these hazards disconnect wetlands from adjacent uplands, yet their hydroecological effects remain poorly understood. Using continuous hydrological observations in Chongming Island, Changjiang Estuary, China, we compared seaward and landward wetlands separated by a seawall. CTD sensor data, water quality measurements, and plot surveys revealed that seawall construction weakens tidal connectivity and reduces hydrological exchange landward. Seaward wetlands exhibit tidal‐dominated dynamics with higher salinity and lower chlorophyll (CHL), chemical oxygen demand (COD), and total organic carbon (TOC), associated with presence of Phragmites australis. Interior wetlands show reduced salinity, higher values of CHL, COD and TOC, stronger rainfall‐driven responses, and are characterized by more terrestrial vegetation species. These findings suggest that seawalls can substantially alter wetland hydrodynamics and ecosystem structure, highlighting the importance of incorporating hydrological connectivity into seawall design to sustain wetland functions.