The psychology of regret has established that a well-justified decision with negative consequences is regretted less, ceteris paribus, than an ill-justified decision of comparable consequences. The question then arises of whether this effect is direct (as Decision Justification Theory would appear to suggest), or indirect – and in that case, what could be potential mediators of the effect. We suggest mediation by counterfactual mutation, that is, the mental simulation of a world where the decision would not have been made. We hypothesise that a good justification makes that simulation more difficult, which in turn decreases regret. An experiment conducted on a sample of more than 900 participants, using the routine-deviation paradigm, shows that this mediation exists but is only partial. Justifications thus have a twofold effect on regret, at the same time direct and indirect. This result sharpens our understanding of the emotion of regret, and call for refinements of Decision Justification Theory.