Abstract Background and aims Individuals with schizophrenia may often experience poor sleep, self-stigma, impaired social functions, and problematic smartphone use. However, the temporal relationships between these factors have not been investigated. The present study used a longitudinal design to examine potential mediating roles of poor sleep and self-stigma in associations between problematic smartphone use and impaired social functions among individuals with schizophrenia. Methods From April 2019 to August 2021, 193 individuals with schizophrenia (mean [SD] age = 41.34 [9.01] years; 88 [45.6%] males) were recruited and asked to complete three psychometric scales: the Smartphone Application-Based Addiction Scale to assess problematic smartphone use; the Pittsburgh Sleep Quality Index to assess sleep quality; and the Self-Stigma Scale-Short Scale to assess self-stigma. Social functioning was evaluated by a psychiatrist using the Personal and Social Performance Scale. All measures were assessed five times (one baseline and four follow-ups) at three-month intervals between assessments. Results General estimating equations found that problematic smartphone use (coefficient = −0.096, SE = 0.021; P < 0.001), sleep quality (coefficient = −0.134, SE = 0.038; P < 0.001), and self-stigma (coefficient = −0.612, SE = 0.192; P = 0.001) were significant statistical predictors for social functioning. Moreover, sleep quality and self-stigma mediated associations between problematic smartphone use and social functioning. Conclusion Problematic smartphone use appears to impact social functioning longitudinally among individuals with schizophrenia via poor sleep and self-stigma concerns. Interventions aimed at reducing problematic smartphone use, improving sleep, and addressing self-stigma may help improve social functioning among individuals with schizophrenia.