Recent studies have shown that inducing people not to believe in free will creates a feeling of helplessness and greatly diminishes prosocial behavior. In fact, people are more likely to cheat in an academic test or to be aggressive towards other individuals. Since altruism is a strong manifestation of prosocial behavior, we test if belief in free will can have an influence on giving behavior. For that purpose, we designed three distinct treatments which were implemented through a survey experiment, namely one where the notion of free will was reinforced, another where the notion of free will was refuted, and another under a neutral context. Results for giving behaviour in a hypothetical dictator game were compared. We found that on average respondents were less generous under the treatment refuting the existence of free will than in the pro-free will treatment or in the neutral treatment. Furthermore, we found that when a relationship between belief in free will and amounts given in the dictator game existed, it was just for those individuals who reported higher beliefs in free will (using the FAD-Plus scale). We also found a positive and significant relationship between belief in scientific determinism (a subscale of the complete FAD-Plus scale) and generosity.