People’s affective or emotional state can alter their cognitive processing, biasing interpretation of ambiguous stimuli. Those in a more positive state interpret such stimuli in a more optimistic manner than those in a negative state. Recently this research has extended to animals, and has shown that manipulations associated with negative affect cause animals to interpret ambiguous stimuli more pessimistically. We investigated whether exposure to environmental enrichment engenders optimistic responses to ambiguous stimuli. Rats, Rattus norvegicus, were trained on a novel conditional discrimination task whereby they learned the correct response necessary to obtain a food reward given the stimulus present during approach (rough or smooth sandpaper). One stimulus was associated with a higher-value reward than the other. Once the rats were trained, cognitive bias was probed by exploring their responses to an ambiguous stimulus (intermediate grade of sandpaper); a rat was defined as optimistic if it chose the response appropriate to the stimulus associated with the better reward. Animals transferred from unenriched to enriched cages showed more optimistic responses following the change. A control group maintained in unenriched cages showed pessimistic responses throughout. These results demonstrate for the first time that environmental enrichment can induce an optimistic cognitive bias in rats previously housed in standard caging, possibly indicative of a more positive affective state. These results add support to the suggestion that measuring cognitive biases can give an insight into animal emotional states; this has implications for animal welfare and preclinical testing of potential therapeutics for mood disorders.