The objective of this work is to determine the relative importance of different processes (swash, overwash, aeolian, and lagoon) affecting sediment dynamics in barriers with frequent overwash. Aerial photographs, topographic surveys (with real-time kinematic global positional system), and sample collection were undertaken on Barreta Island (Ria Formosa barrier islands, Portugal). Wave and tidal data were also analysed during a 3-year monitoring period (2001–2004). This 3-year period represents the typical wave conditions of this region. Historical barrier evolution was characterised for the 1947–2001 period. Up until 1996, the supratidal barrier changed from barred sand to fully developed dunes. In 2001, a wide washover plain had developed and an updrift tidal inlet (Ancão Inlet) was migrating toward the study area. Between 2001 and 2004, the study area had three geomorphological sectors. Sector A (westernmost) showed the widest beach berms that developed because of welding of swash bars of Ancão Inlet. Sector B had the widest washover and developed embryonic dunes with well-sorted medium sands. In sector C (easternmost), overwash was more frequent, and induced the deposition of 57 m³/m of sediments (the coarsest of the barrier). Globally, barrier volume variation was mostly due to swash (83%); however, net barrier accretion was 88% induced by overwash. Barrier volume variation was more related to Ancão Inlet evolution than to the seasonality of wave climate. A four-stage conceptual model of barrier evolution under frequent overwash is presented: beach erosion; dune destruction, and formation of washover plain; frequent overwash; and dune development.