Buoyant trench sediment generates an isostatic restoring force, which opposes the slab pull of subducting plates, but the influence of this force on subduction dynamics is poorly understood. Here, we performed 3D free subduction simulations adopting a variety of sediment distributions along a trench to investigate the correlation between trench kinematics and heterogeneous buoyant force. Two endmembers, sediment-rich and sediment-starved centers, induced convex and concave trenches, respectively. The trench curvatures obtained from natural subduction zones are well constrained by our models over wide magnitudes (1-9 km) and wavelengths (100-400 km) of deficient and excess sediment. Conversely, uniform sediment distribution leads to an extremely narrow range of trench curvature. These results imply that trench sediment contributes to the diversity of trench shapes. Finally, we observed stress localization in slab hinges beneath abundant sedimentation, thereby clarifying the relationship between sediment thickness and subduction earthquakes.