The passage of a tropical storm, as the main driver of storm surge and high waves in many coastal regions, can also generate heavy rainfall and cause river overflow. The resulting combination of riverine, pluvial, and coastal flood hazard can result in catastrophic losses particularly in densely populated coastal environments. In this study, we characterize compound flooding caused by Tropical Storm Matthew and assess the significance and associated uncertainties of multiple contributing factors over a data-scarce coastal region. A hydrological model combined with a simplified two-dimensional hydrodynamic model are set up and validated to investigate the compounding effects of storm tide, wave runup, rainfall, and river overflow at the southern coast of Saint Lucia in the Caribbean Sea. Pléiades-1 and Sentinel-1 satellite imageries are used to determine the flood-impacted areas. The analyses are performed based on deterministic and probabilistic approaches and the effects of uncertain boundary conditions and model parameters are investigated. Results show that the individual analysis of flood hazards, in isolation, can lead to substantial underestimation of flood risks. Heavy rainfall and wave runup are the most significant contributors to compound flooding in Saint Lucia. In addition, the interactions between seawater and streamflow can exacerbate riverine flood hazards particularly upstream of the river mouth. Communities in western Vieux Fort, and the Hewanorra International Airport, have high exposure to compound flooding, which is projected to intensify under climate change.