The tides of the Qiantang River in eastern China are one of the three major tides in the world. Although these tides are spectacular, they seriously threaten the seawalls of the river. Rapid and accurate monitoring of ground deformations along the seawalls is important not only to the seawalls themselves but also to the vast amount of land behind them. We carried out comprehensive, unprecedented high-density mapping of ground deformations along the seawalls of the Qiantang River using the latest full scatterer (FS) interferometric synthetic aperture radar (InSAR) technique with 56 Sentinel-1 SAR images acquired from October 2020 to October 2022. The InSAR-derived deformations were then validated with quasi-synchronous leveling measurements. The results demonstrate that there are five subsidence centers on the northern seawalls and three on the southern seawalls, with a maximum subsidence rate of 96 mm/a and a measuring density of 1293 points/km2. The main reasons for the severe subsidence are the natural consolidation of soft soil in reclaimed areas, groundwater overexploitation due to aquaculture, and foundation pit drainage. We present, for the first time, a comprehensive mapping of ground deformations along the seawalls of the Qiantang River at ultrahigh density, which is very important for the health management of seawalls and the safety of the river on both sides.