The internal structure of a modern baymouth barrier is analyzed in detail, including its historical evolution and the factors controlling it. Historical evolution was reconstructed using available aerial photographs from 1956 to 2003. Detailed geophysical data were acquired using ground-penetrating radar (GPR) and analyzed along with airphotos to reconstruct the recent evolution of a barrier located in northwestern Spain. The GPR data reveal the internal architecture of the barrier, which consists of two main radar facies: (1) washover deposits associated with landward barrier migration and vertical aggradation, and (2) beach deposits associated with seaward barrier progradation. The reconstruction of the stratigraphic sequence indicates the occurrence of two erosive periods followed by an accretionary period. The erosion along the barrier is outlined by the presence of three well-defined bounding surfaces. The stratigraphic record of the subsequent recovery phase depends on the impact level of the erosive phase, which is controlled by the presence of ephemeral inlets along the barrier. This alternating behavior implies a discontinuous shoreline trajectory in spite of a net landward migration. This trend is consistent with regional sea-level rise albeit being intensified locally by the interplay of sediment starvation and storm regime. Landward barrier retreat occurs by overwash and inlet breaching, which permits barrier preservation of volume due to a continuous constructive-destructive cycle. The results acquired from this example expand our understanding of the internal structure and evolutionary trends of transgressive sand barriers.