The ability of the emotion gratitude to shape costly prosocial behavior was examined in three studies employing interpersonal emotion inductions and requests for assistance. Study 1 demonstrated that gratitude increases efforts to assist a benefactor even when such efforts are costly (i.e., hedonically negative), and that this increase differs from the effects of a general positive affective state. Additionally, mediational analyses revealed that gratitude, as opposed to simple awareness of reciprocity norms, drove helping behavior. Furthering the theory that gratitude mediates prosocial behavior, Study 2 replicated the findings of Study 1 and demonstrated gratitude's ability to function as an incidental emotion by showing it can increase assistance provided to strangers. Study 3 revealed that this incidental effect dissipates if one is made aware of the true cause of the emotional state. Implications of these findings for the role of gratitude in building relationships are discussed.