Background: Breast cancer is a well-recognised late adverse effect in female childhood cancer survivors (CCSs), especially after chest radiotherapy; information on subsequent male breast cancer (SMBC) is limited. We summarised the existing evidence on SMBC after childhood cancer in a systematic review and investigated the risk of SMBC among males in a Pan-European cohort. Methods: We searched Medline/PubMed for cohort studies and case reports/series that assessed SMBC after childhood cancer (≤21 years). Furthermore, we analysed data on SMBC in the PanCareSurFup cohort, reporting standardised incidence ratios (SIRs), absolute excess risks (AERs), and 5- and 10-year survival rates. Results: The systematic review included 38 of 7080 potentially eligible articles. Cohort-specific SMBC frequencies were 0–0.40% (31 studies). SMBC occurred after a follow-up ranging from 24.0 to 42.0 years. Nine case reports/series described 11 SMBC cases, occurring 11.0–42.5 years after primary childhood cancer. In the PanCareSurFup cohort (16 SMBC/37,738 males; 0.04%), we observed a 22.3-fold increased risk of SMBC relative to the general male population (95% CI 12.7–36.2; absolute excess risk/100,000 person-years: 2.3, 95% CI 1.3–3.7). The five- and ten-year survival rates after SMBC diagnosis were 60.3% (95% CI 35.6%–85.0%) and 43.0% (95% CI 16.1%–69.9%), respectively. Clear evidence of risk factors did not emerge from these comprehensive efforts. Conclusions: Compared to the general population, male CCSs have an elevated risk of developing subsequent breast cancer, although the absolute risk is low. Health care providers should be aware of this rare yet serious late effect; male CCSs with symptoms potentially related to SMBC warrant careful examination.