River plumes are transition zones where freshwater discharged from river mouths enters and mixes with ambient seawater, creating low-density surface layers that can extend tens to hundreds of kilometers offshore. Plume morphology and the frontal zones bounding plume waters strongly influence the transport and spatial distribution of river-borne materials. Accurately identifying plume extent and frontal structures is therefore essential for understanding the spatial distribution of biogeochemical tracers and connectivity between sources (river mouths) and offshore regions. We investigated the spatial extent and dynamics of Vietnam’s largest river plume, the Mekong River plume (MRP), using satellite-derived sea surface topography from the Surface Water and Ocean Topography (SWOT) mission, together with ocean color and in situ measurements from the PLUME oceanographic campaign conducted in June 2024. We employed a variational method of velocity interpolation to mitigate errors associated with noisy and gappy SWOT sea surface height (SSH) data and their propagation into velocity estimates. The complexity of the SSH signal reflects large buoyancy input from multiple estuaries, generating highly energetic circulation in this shallow water region, further influenced by strong wind forcing, with surface currents often exceeding 1 m s⁻¹. Analysis of SWOT-derived velocities revealed coherent surface circulation patterns and the presence of submesoscale frontal structures within the plume body. Frontal zones were identified by extracting Lagrangian Coherent Structures (LCS) from the surface velocity field using finite-size Lyapunov exponents (FSLE). Large values of FSLE, correspond to ridges in the attracting FSLE field, delineate zones of enhanced shear along outflowing manifolds, revealing diverse frontal geometries and enabling characterization of plume frontal systems and offshore expansion. The results of plume characterization from remote sensing were supported by comparisons with velocities obtained from surface drifter trajectories and acoustic Doppler current profiler (ADCP) measurements. The convergence of surface drifters within frontal zones identified from SWOT measurements provides evidence that, in the study region, SWOT-derived velocities at the submesoscale are not in geostrophic balance. In regions dominated by submesoscale eddies, the flow becomes subgeostrophic and is often better approximated by the gradient flow, a three-way balance among the Coriolis, centrifugal, and pressure-gradient forces. In such flows, convergence and divergence can occur. These results have important implications for future assessments of horizontal dispersion, vertical motions, and mixing processes in river plume systems with major ecological and economic significance in coastal ocean regions.