The present study examined weight gain in a stressful environment (i.e., following compulsory enrollment in the Greek army). It was predicted that higher levels of mindfulness and self-compassion would relate negatively to weight gain, whilst negative automatic thoughts and intolerance of uncertainty would positively relate to weight gain. This research also explored the strength of independent variables when predicting weight gain, plus the additional contribution of self-compassion when controlling for the effect of mindfulness on weight gain. Ninety-seven military recruits completed the psychological scales on the first day of enrollment. Their height and weight were measured at baseline and after 5 weeks to record weight gain. Results revealed that 43 participants gained weight, whilst 54 lost weight. Those who lost weight reported significantly higher scores in mindfulness and self-compassion, whereas those who gained weight reported significantly higher scores in negative automatic thoughts and intolerance of uncertainty. Furthermore, negative automatic thoughts and intolerance of uncertainty did not significantly predict weight gain, after mindfulness and self-compassion were taken into account. Also, self-compassion uniquely contributed to the negative prediction of weight gain, once mindfulness was taken into account. This research concluded that negative cognition may play a role in weight gain; however, mindfulness and self-compassion may be more useful traits in predicting weight gain, given that once they are taken into account, negative cognition stop being significant predictors.