Ocean warming and acidification are two important environmental drivers affecting marine organisms. Organisms living at high latitudes might be especially threatened in near future, as current environmental changes are larger and occur faster. Therefore, we investigated the effect of hypercapnia on thermal tolerance and physiological performance of sub-Arctic Mytilus edulis from the White Sea. Mussels were exposed (2 weeks) to 390 µatm (control) and 1120 µatm CO₂ (year 2100) before respiration rate (MO₂), anaerobic metabolite (succinate) level, haemolymph acid–base status and intracellular pH (pHi) were determined during acute warming (10–28 °C, 3 °C over night). In normocapnic mussels, warming induced MO₂ to rise exponentially until it levelled off beyond a breakpoint temperature of 20.5 °C. Concurrently, haemolymph PCO₂ rose significantly > 19 °C followed by a decrease in PO₂ indicating the pejus temperature (T_P, onset of thermal limitation). Succinate started to accumulate at 28 °C under normocapnia defining the critical temperature (T_C). pHi was maintained during warming until it dropped at 28 °C, in line with the concomitant transition to anaerobiosis. At acclimation temperature, CO₂ had only a minor impact. During warming, MO₂ was stimulated by CO₂ resulting in an elevated breakpoint of 25.8 °C. Nevertheless, alterations in haemolymph gases (> 16 °C) and the concomitant changes of pHi and succinate level (25 °C) occurred at lower temperature under hypercapnia versus normocapnia indicating a downward shift of both thermal limits T_P and T_C by CO₂. Compared to temperate conspecifics, sub-Arctic mussels showed an enhanced thermal sensitivity, exacerbated further by hypercapnia, indicating their potential vulnerability to environmental changes projected for 2100.