Assessing welfare of horses, which is a particularly critical point in terms of ethics, safety and performance, is still under debated. Although, clear behavioral and postural indicators have been validated, their use remains restricted to persons trained to observational techniques and the physiological parameters used so far gave contradictory results. Furthermore, these indicators concern particularly horses' ill-being. Therefore, in order to assess clearly and simply horses' welfare we need objective markers that can be used by non-specialists. Recently, it has been shown that a horse in a good welfare level seems to be quiet and attentive to its environment. The telemetric EEG headset developed in our lab allowed us to show that horse attentional state can alter the EEG profile (proportion of brain waves). In the light of these results we hypothesize that welfare may also alter EEG profile and that EEG profile may become a good objective physiological marker of welfare. In order to test this hypothesis, we performed EEG recordings on two populations of horses living in two different environment and presenting different welfare levels. One population was living in riding center with restricted conditions (single stalls, limited access to roughage…) and the other population was living in naturalistic conditions (stable groups, pasture with grass or hay adlibitum…). The welfare of these horses was precisely evaluated using behavioral markers. We recorded separately the EEG of the left and right hemispheres of the horses while they were quietly watching their environment. We then built the individual EEG profiles (proportion of the different brainwaves) of left and right hemisphere. The results show a clear difference of well-being of both populations, horses living in naturalistic conditions presented a well-being state clearly better than the horses living in riding center. The EEG profiles varied as a function of the population and were correlated to horse well-being. These results show that EEG may become a new objective marker of welfare in horses.