Recent years have seen the development of 1-D and 2-D regional-scale hydrological-hydrodynamic models, which differ greatly from reach-scale applications in terms of subgrid assumptions, parameterization, and applied resolution. Although 1-D and 2-D comparisons have already been performed at reach and local scales, model differences at regional scale are poorly understood. Moreover, there is a need to improve the coupling between hydrological and hydrodynamic models. It is addressed here by applying the MGB model at 1-D and 2-D dimensions for the whole ~700,000 km2 Negro basin (Amazon), which presents different wetland types. Long-term continuous simulations are performed and validated with multisatellite observations of hydraulic variables. Results showed that both approaches are similarly able to estimate discharges and water levels along main rivers, especially considering parameter uncertainties, but differ in terms of flood extent and volume and water levels in complex wetlands. In these latter, the diffuse flow and drainage patterns were more realistically represented by the 2-D scheme, as well as wetland connectivity across the basin. The 2-D model led to higher drainage basinwide, while the 1-D model was more sensitive to hydrodynamic parameters for discharge and flood extent and had a similar sensitivity for water levels. Finally, tests on the coupling between hydrologic and hydrodynamic processes suggested that their representation in an online way is less important for tropical wetlands than model dimensionality, which largely impacts water transfer and repartition.