The purpose of this study is to verify the mediating effect of grit in the structural relationship between social support (parent support, teacher support, and peer support) and happiness perceived by middle school students, and to examine the difference of the effect between poverty and non-poverty groups. For this purpose, data from the third year of the Korea Child and Youth Panel Survey (KCYPS) 2018 middle school panel (third-graders) conducted by the National Youth Policy Institute (NYPI) were used. Structural equation modeling (SEM) analysis and multiple-group analysis of poverty and non-poverty groups were conducted on a total of 2,380 participants (1,274 male, 1,106 female). The results showed that, first, parental support partially mediated grit and had a positive effect on happiness. Second, teacher support completely mediated grit and had a positive effect on happiness. However, the mediating effect of grit was not significant in the relationship between peer support and happiness. Third, significant differences between the poverty and non-poverty groups were revealed only in the area of peer support on happiness. Specifically, the poverty group perceived happiness from peer support 1.8 times greater than that of the non-poverty group. This study is meaningful in that it reveals the structural relationship according to types of social support by analyzing poverty and non-poverty groups separately, and suggests educational policy measures that can improve the happiness of middle school students.