This research extends the existing theoretical understanding of what predicts organizational citizenship behavior (OCB). Using experience sampling techniques, we examine the within-person relation between OCB and a novel, theoretically relevant predictor: state gratitude. Using 4 independent samples with a total of 210 working adults and 173 undergraduate students, we developed a reliable and valid measure of state gratitude. Drawing upon the moral affect model of gratitude and affective events theory, we conducted 2 experience sampling studies with data collected from 67 (Study 2) and 104 (Study 3) working adults to test the effects of state gratitude on OCB, beyond the effects of several relevant constructs (i.e., state positive affect, dispositional gratitude, and social exchange). Our results advance OCB research and explanations of OCB by modeling OCB as a dynamic, time-variant construct and by demonstrating that feelings of gratitude, a discrete positive emotion, can be an effective predictor of OCB.