Coastal areas contain interfaces between complex depositional systems as a consequence of the convergence of marine and continental environments. The knowledge of such complex depositional systems is important in understanding of coastal evolution and for coastal management. The present study deals with the subsurface identification of interfaces between fluvial and beach/shallow marine systems, using Ground Penetrating Radar (GPR). GPR profiles were measured along dip direction, at two sites on the coastal plain of Santa Catarina state (Pinheira and Passo de Torres), where deposits of fluvial and beach/foredune ridge systems occur. GPR data showed that fluvial and beach deposits are interdigitated in subsurface, with reflectors showing different downlap directions. The results of this study allow a more complete knowledge of the complex interfaces between marine and continental systems. Also, the identification of hidden older fluvial deposits in the subsurface of coastal plains gives important geotechnical information to be considered in the use and occupation of coastal areas and during prospection for placer deposits.