The fitness consequences of outbreeding in wild populations are extremely variable. Heterosis and outbreeding depression can both be observed but the effect of environmental stressors on the occurrence of these phenomena is still poorly understood. We tested the influence of oxygen stress during embryonic development on consequences of outbreeding in wild populations of Atlantic salmon (Salmo salar). We used a common garden experiment and performed crosses within and between salmon populations to study performances of embryos reared under normal and hypoxic conditions. We detected both heterosis and outbreeding depression depending on traits but irrespective of divergence between parental populations. Nevertheless, outbreeding depression was observed almost exclusively under hypoxic conditions and prevailed over heterosis regarding survival during the whole embryonic development. Notably, the post-hatching survival of all between population crosses was approximately 15 % lower than the survival of within-population crosses under hypoxic conditions. Different hypoxia reaction norms for post-hatching survival, length and time to hatch were also noticed among within and between populations crosses further indicating outbreeding depression. These results demonstrate that consequences of outbreeding can dramatically vary depending on environmental conditions with outbreeding depression being possibly stronger under stressful conditions.