A diesel reformer start-up strategy using hydrogen peroxide (H2O2) decomposition heat is first proposed for subsea applications such as submarines and unmanned underwater vehicles (UUVs), requiring no oxygen. H2O2 can supply not only both the oxygen and steam required for the diesel autothermal reforming (ATR) reaction but also heat via the catalytic H2O2 decomposition reaction. We utilize the high-quality heat from H2O2 for the reformer start-up by evaporating and chemically decomposing the 50 wt% concentration H2O2. A comparative reformer start-up strategy study is performed between conventional diesel ATR and diesel-H2O2 ATR using a 1 kW(e)-class diesel reformer. For the conventional start-up, three steps of partial oxidation, partial load ATR, and full load ATR are required to heat the ATR reformer after electrically preheating the ATR catalyst. However, the proposed strategy requires only one full load ATR step after preheating the ATR catalyst using H2O2 decomposition heat. With the suggested start-up strategy, a simplified sequence with negligible temperature fluctuations and a 57% start-up time reduction are achieved. We find it significant that the full load flowrate of superheated steam generation from the H2O2 decomposer is a critical factor in simplifying and shortening the start-up procedure.