Submesoscale filaments are a well-documented feature of oceanic flow fields. Based on the analysis of high-resolution 3D numerical simulations, field observations and remote sensing imagery, this study, carried out in Lake Geneva, Western Europe's largest lake, provides for the first time evidence that submesoscale filaments also exist in a lake. Field observations confirm that submesoscale, cold-water filaments are formed at the edges and in the center of a counterclockwise rotating large-scale gyre during summertime above the thermocline, as indicated by 3D numerical simulation results and remote sensing. Cold submesoscale filaments were well-developed with sharp lateral temperature gradients (~1-2°C) during summertime. Locally, these filaments can significantly increase the depth of the upper mixed layer when a relatively strong thermocline exists in the near-surface layers and cause upward and downward vertical velocities comparable to those reported for oceanic filaments.