Animals’ affective states can be assessed through judgement bias tests, evaluating responses to ambiguous situations. In this study, Rainbow trout (n=108) were housed in commercial conditions while trained at tank-level to discriminate between a positively reinforced chamber (feed) in one location and a negatively reinforced chamber (chase by net) in the opposing location. Fish from successful tanks (2/5) were then housed in treatment tanks of either high or low environmental complexity at either high (192 fish/m3) or low (80 fish/m3) stocking density. Trained fish were tested for latencies to approach three intermediate, ambiguous chambers. Fish housed in high-density tanks were faster to enter all chambers than those housed in low-density tanks (8.5s vs. 15.2s; p=0.001), with faster entries into the positive (7.4s vs. 15.2s; p=0.02) and near-negative chambers (10.2s vs. 17.4s; p=0.006), suggesting that these fish were more optimistic to receive a feed reward. Tank complexity did not affect test outcomes. Body weight, length, and plasma cortisol did not differ between treatments. Overall, Rainbow trout are capable of discriminating between cues during a judgement bias test and fish housed in high-density environments respond more optimistically in ambiguous situations compared to fish in low-density environments.