A new magnetic field generation mechanism in electrostatic shocks is found, which can produce fields with magnetic energy density as high as 0.01 of the kinetic energy density of the flows on time scales ∼10 4 ω −1 pe . Electron trapping during the shock formation process creates a strong temperature anisotropy in the distribution function, giving rise to the pure Weibel instability. The generated magnetic field is well confined to the downstream region of the electrostatic shock. The shock formation process is not modified, and the features of the shock front responsible for ion acceleration, which are currently probed in laserplasma laboratory experiments, are maintained. However, such a strong magnetic field determines the particle trajectories downstream and has the potential to modify the signatures of the collisionless shock. Collisionless shocks have been studied for many decades, mainly in the context of space and astrophysics [1][2][3][4]. Recently, shock acceleration raised significant interest in the quest for a laser-based ion acceleration scheme due to an experimentally demonstrated high beam quality [5][6][7][8].Interpenetrating plasma slabs of hot electrons and cold ions are acting to set up the electrostatic fields via longitudinal plasma instabilities. The lighter electrons leaving the denser regions are held back by the electric fields, which pull the ions. Particles are trapped in the associated electrostatic potential, which steepens and eventually reaches a quasisteady-state collisionless electrostatic shock. Most of the theoretical work dates back to the 1970s [9-13] relying on the pseudo-Sagdeev potential [14] and progress has been mainly triggered by kinetic simulations [15][16][17][18].The short formation time scales and the one dimensionality of the problem make it easily accessible with theory and computer simulations. However, long time shock evolution was often one dimensional or electrostatic codes were used, and the role of electromagnetic modes was mostly neglected. More advanced multidimensional simulations have shown the importance of electromagnetic modes also in this context, due to transverse modes which are excited on the ion time scale [19,20]. We show that in the case of very high electron temperatures associated with the formation of electrostatic shocks [21], electromagnetic modes become important on electron time scales, creating strong magnetic fields in the downstream of the shock.With the increase in laser energy and intensity, the possibility to drive electrostatic shocks has become important for laboratory experiments of electrostatic shocks. Recent laser-driven shock experiments showed the appearance of an electromagnetic field structure [22][23][24], which was attributed to the ion-filamentation instability [25] that evolves on time scales of ten thousands of the inverse electron plasma frequency, ω −1 pe . As a main outcome of this Letter, we show that these structures can already be seeded and produced on tens of ω −1 pe and remain in a quasisteady state over t...