Land and marine ERT data set collected respectively in low and high tide conditions. An overlapping exists in the intertidal zone. This data set is used as a demonstration example of the combined inversion of land and marine ERT. The data correspond to land and marine ERT data collected in the Western Belgian coastal plain next to the French border. This site has been largely studied in the past for its characteristic distribution of fresh- and saltwater. The large recharge in the dune area located to the South, combined with the low slope (1.1%) of the beach and the large tidal range (about 5 m) is responsible for the presence of a saltwater lens above fresh water in the intertidal zone. The total profile is about 800 m long, with 102 electrodes spaced every 5 m, for a total of 505 m on land (limit of the low water line). Land data were collected using a multiple-gradient array at low tide (-0.15 m). There is a 200 m overlap with the marine profile, collected at high tide (3.41 mTAW), which continues 300 m further offshore. The marine data were collected through continuous resistivity profiling, using 13-electrode cables with 15 m spacing and a reciprocal Wenner-Schlumberger array. Data were collected on average every 3.2 m, together with the height of the water column (bathymetry).