Background and aims: Aquatic exercises are popular leisure activities worldwide, primarily among women. These activities are especially beneficial for aging people and individuals having difficulties performing land-based exercises. Their physical health benefits have already been documented in the academic literature, but research on their mental health effects is still non-existent. However, leisure exercises promoting mental health are advantageous in a fast-paced and often stressful world. Therefore, in this in-situ (natural life setting) field study, we examined the acute mental benefits of aquatic exercises in 30 voluntary consenting women having a mean age of 57.57 (SD = 12.67) years. Methods: Using a within-participants research design, the subjectively perceived feeling states and felt arousal, along with positive and negative affect, were recorded before and after exercise. Moreover, the personal expectancies regarding the expected feelings after exercise were assessed before the workout to determine the influence of anticipation effects. Results: The findings revealed that core affect and positive affect improved substantially from before to after exercise (p < .001), as confirmed by the large effect sizes (Cohen’s d > 0.80). Negative affect decreased nonsignificantly (p = .062), but it was already low before exercise. Although expectancy scores were high before the exercise class, they did not correlate (p > .05) with the dependent measures’ magnitude of changes (pre-class – post-class scores). Conclusions: These findings suggest that women who participate in an aquatic exercise class report experiencing significant positive changes in their feelings, demonstrating this popular exercise’s acute mental health benefits. Although further controlled research is needed in this area, the current results have promising implications for middle-aged women’s mental health preservation and promotion.