<p>Parental phubbing has emerged as a significant risk factor for problematic smartphone use among adolescents, yet the underlying mediating and moderating mechanisms remain insufficiently understood. This study investigated (a) whether loneliness mediates the association between parental phubbing and problematic smartphone use and (b) whether the parent–child relationship with the one parent can buffer the adverse effects of the other parent’s phubbing on adolescent loneliness, and subsequent problematic smartphone use. A three-wave longitudinal design was employed with 657 Chinese adolescents (348 boys, 309 girls; baseline <i>M</i>_age = 13.83 years), with each wave spaced six months apart. Participants completed measures of father/mother phubbing, loneliness, father–child/mother–child relationship, and problematic smartphone use. Moderated mediation analyses were conducted using the PROCESS macro in SPSS 27.0. Results revealed that: (a) both father phubbing and mother phubbing were positively associated with adolescent problematic smartphone use; (b) loneliness partially mediated these links; and (c) the parent–child relationship moderated the mediating pathway such that the association between loneliness and problematic smartphone use was stronger among adolescents reporting higher-quality parent–child relationship, which supports the reverse stress-buffering model, rather than the stress-buffering model. These findings enrich the understanding of the complex pathways through which parental phubbing is linked with adolescent problematic smartphone use and offer practical implications for prevention and intervention strategies.</p>