From 2015 to 2018 four field experiments (seven legs) were performed in the Western Mediterranean basin during winter or early spring. The main objectives were the assessment of high-resolution modeling and the observation of mesoscale structure and associated ageostrophic dynamics. Thanks to the intensive use of a towed vehicle undulating in the upper oceanic layer between 0 and 400 m depth (SeaSoar), a large number of very high resolution hydrographic transects (total length about 10 000 km) were measured, observing mesoscale dynamics (slope current and its instabilities, anticyclonic eddies, submesoscale coherent vortices, frontal dynamics, convection events, strait outflows) and submesoscale processes like stirring, mixed-layer or symmetric instabilities. When available, the data were completed with velocities recorded by a vessel-mounted acoustic Doppler current profiler (VMADCP) and by surface salinity and temperature recorded by a thermosalinograph (TSG). Classical full-depth CTD (conductivity, temperature, depth) stations were also used, providing the background hydrography of the deeper layers when focusing on peculiar structures. In 2017, a free-fall profiler (MVP-200; MVP – moving vessel profiler) was deployed to manage even higher horizontal resolutions. In 2018, another free-fall profiler (a rapidCAST) was tested. When available, biological sensors (chlorophyll a, turbidity, dissolved oxygen, etc.) were used. They provided useful complementary observations about the circulation. This dataset is an unprecedented opportunity to investigate the very fine scale processes, as the Mediterranean Sea is known for its intense and contrasting dynamics. It should be useful for modellers (who reduce the grid size below a few hundred meters) and should properly resolve finer-scale dynamics. Likewise, theoretical work could also be illustrated by in situ evidence embedded in this dataset. The data are available through the SEANOE repository at: https://doi.org/10.17882/62352 (Dumas et al., 2018).