Abstract : One nutritional problem that is prone to occur in adolescents is malnutrition. Underweight or underweight is a condition caused by a lack of energy intake compared to the energy expended. Lack of energy intake or insufficient consumption of essential nutrients the body needs will usually cause a decrease in the activities carried out. This study aims to assess the effect of social cognitive theory-based nutrition education on behavioral aspects related to physical activity in malnourished adolescents in the Makassar City Island area. This type of research is quasi-experimental with a pre-test and post-test design with a control group design. Using simple random sampling with a total sample of 82 malnourished adolescents on Kodingareng Island, Barang Caddi Island, Barrang Lompo Island, and Lae-late Island in Makassar City, the intervention group and the control group. The variables in this study were looking at changes in behavioral aspects (knowledge, self-efficacy, motivation) related to physical activity in the intervention group (given nutrition education using modules) and the control group (given infographics) for 12 weeks. Data were analyzed using Chi-Square, Wilcoxon Signed Rank Test, and Mann-Whitney. The results showed that there was a change in knowledge in the intervention group after education (8.09 ± 1.20) with a p-value (0.000), as well as in the control group showing there was a change in knowledge after education (7.58 ± 1.53) with a p-value (0.000). The average change in knowledge did not differ between the two groups, with a p = 0.282. There was a change in self-efficacy in the intervention group after education (53.26 ± 15.55) with a p-value (0.000), as well as in the control group showing a change in self-efficacy after education (47.51 ± 19.84) with a p-value (0.000). There was a big difference in the mean change in self-efficacy between the two groups, with a p = 0.009. There was a change in motivation in the intervention group after education (30.75 ± 4.28) with a p-value (0.000), as well as in the control group showing a change in motivation after education (26.58 ± 6.84) with a p-value (0.000). There was a big difference in the average change in motivation between the two groups, with a p = 0.000. There was a change in physical activity behavior in the intervention group after education (1.434±0.134) with a p-value (0.000), as well as in the control group showing changes in physical activity behavior after education (525.63±133.59) with a p-value (0.000). There was a difference in the average change in physical activity behavior between the two groups, with a p = 0.011. Researchers conclude that educational interventions have an effect on behavioral aspects (knowledge, self-efficacy, motivation, physical activity behavior) related to physical activity in the intervention and control groups.