Social support is an important predictor of the well-being of partners of cancer patients. Those who are caring for minors (well parents) may be in special need of such support. The aim of this study was to evaluate the 4-month impact of a psychoeducational social support intervention, named the Cancer-PEPSONE programme (CPP), on well parents' received and perceived social support, as well as on their psychological distress, quality of life (QOL) and parental capacity. The study was an open randomised controlled trial with a parallel-group design (N = 35; Intervention = 17, Controls = 18). The participants in the intervention group received CPP in their homes. Controls received support as usual. Data were collected in Norway using validated self-report questionnaires. Questionnaires were sent by post, before randomisation (T1), and at three- (T2) and six-month (T3) follow-up. Linear mixed models analyses revealed intervention effects on received (p = 0.04, d = 0.6) and perceived (p = 0.01, d = 1.0) social support, as well as on parental capacity (p = 0.02, d = 1.0), but not on psychological distress and QOL. Social support mediated the relationship between receiving CPP and later psychological distress. CPP may help well parents in maintaining social support and enhancing parental capacity. An improvement in social support may alleviate well parents' psychological distress.