Negative affect is known to cause individuals to interpret ambiguous stimuli ‘pessimistically’, exhibiting an increased expectation of punishment. Here, we hypothesised that horses suffering from poor welfare conditions would show more pessimistic judgement biases compared to horses with a better welfare state. This study involves 34 horses from 3 sites differing in management conditions. On one hand, we had 2 groups (n1= 11; n2=14) of riding school horses living in individual boxes, with restricted feeding and used in daily riding lessons with constraining techniques. On the other hand, we had one group (n3=9) of leisure horses living in more naturalistic conditions and used for occasional relaxed riding. We assumed that the riding school horses would have poorer welfare and hence more negative affective state than the leisure horses. The welfare of the horses was assessed by recording health-related (e.g.chronic pathologies, vertebral problems), behavioural (e.g.stereotypy, aggressive behavior) and postural (e.g.ears position) measures that have been proven to be good indicators of a chronic bad welfare state in domestic horses. All horses were also trained on a spatial judgment task during which they learnt to expect edible food in one location and unpalatable food in another. Judgement bias was tested using three ambiguous locations intermediate between the trained locations. A strong site effect was found: the horses living in the site characterized by the highest levels of behavioural and health-related problems exhibited a clear pessimistic bias when judging ambiguous stimuli, whereas the horses living in more natural conditions, associated with a good welfare state, showed an optimistic bias. This study is the first to reveal a link between poor welfare and pessimistic judgement biases in domestic horses, and reinforces existing data suggesting that horses in some riding schools may suffer from poor welfare.