The COVID-19 pandemic has greatly affected people's mental health. The direct and indirect pathways between social support and suicidal ideation in the period are still unclear. This study explores the pathways from social support to suicidal ideation through resilience and depressive symptoms among undergraduates during the COVID-19 campus lockdown. Methods: During two weeks of the COVID-19 campus lockdown, a total of 12,945 undergraduates at a university in eastern China completed the questionnaire including sociodemographic characteristics, suicidal ideation, social support, resilience, and depressive symptoms. A structural equation modeling (SEM) approach was used to analyze the direct and indirect pathways from social support to suicidal ideation via the mediators of resilience and depressive symptoms. Results: Of the 12,917 undergraduates included in this study, 7.4% (n = 955) reported they sometimes had suicidal ideation, 0.8% (n = 109) reported they often had suicidal ideation, 0.9% (n = 122) reported they always had suicidal ideation, and 13.2% (n = 1704) reported they had depressive symptoms. Social support exerted significant direct (β = −0.058), indirect (β = −0.225), and total (β = −0.283) effects on suicidal ideation; 20.5% of the total effect was direct, and 79.5% was indirect. Social support predicted suicidal ideation through resilience (β = −0.038), and depressive symptoms (β = −0.087), explaining 13.4%, and 30.7% of the total effect, respectively. Social support predicted suicidal ideation through the sequential mediation of resilience and depressive symptoms (β = −0.099), explaining 35.0% of the total effect. Conclusion: This is the first study to provide the evidence of pathways from social support to suicidal ideation through resilience and depressive symptoms during the COVID-19 campus lockdown among undergraduates in China. Both direct and indirect pathways from social support to suicidal ideation were identified as intervention targets to reduce suicidal ideation.