Tavira Island, the largest barrier island on Portugal's Algarve coast, exhibits a broad backbarrier area that probably consists of relict flood tidal deltas that were incorporated when associated tidal inlets closed. The tidal deltas are preserved as lobate landforms extending into the marsh, and are bordered by linear, partially infilled depressions that resemble flood channels on modern tidal deltas in the region. The origin and evolution of the incorporated deltas and the location of former inlets were investigated using a dataset of 6.7 km of ground-penetrating radar (GPR) profiles and more than 30 cores. The data reveal a complex stratigraphic framework including a series of lateral facies changes that are interpreted relative to the incorporated tidal delta model and sedimentary environments as seen in the modern barrier chain.