Objective: The objective of this study was to explore the relationship between extracurricular activities, stress, and suicidal ideation and to examine the mediating effect of stress between extracurricular activities and suicidal ideation in Chinese college students. Method: A total of 6446 college students were surveyed with a web-based online data collection system using the self-made demographic questionnaire, Suicidal Behaviors Questionnaire—Revised (SBQ-R), and the 21-Item Depression Anxiety Stress Scales (DASS-21). SPSS 24.0 was used for descriptive statistics and correlation analysis, and the bootstrap method in the process procedure for SPSS Version 3.4.1 was used to construct the mediating effect model. Results: Gender, school grades, living area, and family income status were influencing factors for suicidal ideation, stress, and extracurricular activities. Extracurricular activities were negatively correlated with stress (r = −0.083, p < 0.001) and suicidal ideation (r = −0.039, p < 0.01). Extracurricular activities had no direct predictive effect on college students’ suicidal ideation (c = −0.198, CI: −0.418, 0.023), while stress had a mediating effect between extracurricular activities and suicidal ideation; the indirect mediating effect was 0.159. Conclusions: Extracurricular activities indirectly predict college students’ suicidal ideation through stress. A variety of extracurricular activities can decrease the stress and suicidal ideation of college students and benefit their mental health.