This study investigated the changing trajectory of grit and the contemporaneous and lagged effects of three sources of social support (parental, peer, and teacher support) in elementary school students. For this purpose, data from Waves 1 through 3 (4th through 6th grades) of the Korean Children and Youth Panel Survey 2018 (KCYPS 2018), conducted by the National Youth Policy Institute, were used. Latent growth modeling was applied to analyze the changing trajectory of grit and the contemporaneous and lagged effects of social support over the three years. The results are as follows. First, students’ grit appeared to decrease through the elementary school years. Second, the contemporaneous effects of the three sources of social support on grit were positively significant in each grade. In addition, significant lagged effects of teacher support in the 4th and 5th grades and on grit in the 5th and 6th grades were found in the male group, while significant lagged effects of teacher support in the 4th grade and on grit in the 5th grade were found in the female group. In sum, elementary school students’ grit tends to decrease as grade level increases. However, the importance of social support from parents, peers, and teachers is confirmed by revealing the contemporaneous and/or lagged effect of social support on the elementary male and female students’ grit. It is suggested that additional longitudinal research on grit be conducted.