<p>Gratitude expressions of recipients toward benefactors prompt third-party witnesses to ascribe moral qualities to benefactors. To date, however, relatively little is known about how observing the gratitude expressions of others toward their benefactors affects moral perceptions of expressers. Given that gratitude is a moral emotion, its expression should signal an expresser’s moral qualities, and possibly by extension, a halo of positive traits outside the moral domain. Here, we measured how the expression of gratitude impacts inferences about expressers’ moral and general positive traits (i.e., halo effects) and how such perceptions shape observers’ desires to affiliate and cooperative behavior in a trust dilemma with expressers. Across two studies (<i>N</i> = 680), participants watched a video depicting a benefactor resolving a problem for an expresser who conveyed either gratitude or simple positivity in return, and then provided impressions of the expresser, and, in the second study, decided whether to cooperate with the expresser to earn money where exploitation by the expresser was possible. We found witnessing gratitude expressions produced stronger moral and nonmoral positive halo effects than witnessing simple positivity expressions and were principally mediated by perceptions of greater responsiveness from expressers. These halos, in turn, increased trust, perceived closeness, and desires to interact with expressers, with trust directly influencing decisions about whether to cooperate on a financial task entailing risk. The findings have implications for the group-level function of gratitude in signaling expresser traits and helping witnesses identify high-quality social partners.</p>