Objective: This study examined the mediating effect of depression in the relationship between defeat and suicidal ideation among middle school students and the moderated mediating effect of adaptive cognitive emotion regulation. Methods: The study participants were middle school students in Seoul, Gyeonggi, and Gyeongbuk. The data were collected using online self-report questionnaires and analyzed using SPSS 27.0 and Process Macro software. Results: First, depression mediated the effect of middle school students’ defeat on suicidal ideation. Second, there was a significant moderated mediating effect in which the effect of defeat on suicidal ideation through depression depended on the level of adaptive cognitive emotion regulation. Conclusions: To prevent suicidal ideation in middle school students, it is necessary to alleviate their irrational sense of defeat and reduce the possibility of defeat leading to depression. In addition, if middle school students are depressed due to their sense of defeat, helping them control their emotions in a cognitive way can be an effective intervention for suicidal ideation.