Social cognitive theory provides a framework of human agency during environmental challenges, with coping self-efficacy (CSE) as an important construct underlying adaptation. We examined two alternative models involving CSE as a mediator of the association between posttraumatic stress symptoms (PTSS) and communal coping among parent-youth dyads after severe floods using Bayesian dyadic multilevel modeling. The first model included PTSS as the independent variable and communal coping as the dependent variable (disaster distress model). The independent and dependent variables were replaced for each other in the second model (communal coping model). We used data from 485 parent-youth dyads who experienced floods between 2015 and 2016 in Texas, USA. Parents of children (69% women) aged 10–19 years old, and their oldest child (53% male; Mean age = 13.75) in that age range were recruited. We assessed PTSS, CSE, and communal coping for parents and youths. Results favored the disaster distress model over the communal coping model. In the disaster distress model, results demonstrated that CSE declines as PTSS increases, predicting decreased communal coping. This mediation effect of CSE is stronger for youths compared to parents, indicating that children’s CSE is affected more by PTSS.