In many places around the world, tide gauges have been measuring substantial non-astronomical changes. Here we document an exceptional large spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between -2.3 mm/yr in the UK and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous, suggesting a superposition of local and large-scale processes at work within the basin. We use principal component analysis to separate large-scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along UK and German coastlines, which suggest movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large-scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland and Canada shows significant correlations on time scales of less than 2 years, which suggests an external and basin-wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.