Kelps, the marine forest foundation species, are threatened by ocean warming at their warm distributional edges. To mechanistically investigate inheritance of thermal traits, we assessed thermal tolerance of inbred (selfings) and outbred (crosses) sporophytes of the N-Atlantic kelp Laminaria digitata among isolates from the genetically distinct populations of Helgoland (North Sea) and Spitsbergen (Arctic). First, we investigated the upper thermal tolerance of microscopic sporophytes in a 14-day experiment applying 20–23°C. The upper survival temperature was lower for the Spitsbergen selfing (21°C) than for the Helgoland selfing and the reciprocal crosses (22°C). We then subjected 4–7 cm long sporophytes to a control temperature (10°C), moderate (19°C) and sub-lethal heat stress (20.5°C) to assess metabolic regulation via whole-transcriptome analysis in addition to physiological parameters. Growth and optimum quantum yield decreased similarly in both crosses and the Helgoland selfing at 19 and 20.5°C, while inbred Spitsbergen sporophytes died within seven days at both 19 and 20.5°C. At 10°C, the Spitsbergen selfing showed the highest differential gene expression. Considering only the three surviving lineages at 20.5°C, differential gene expression was 61–78% lower in the crosses compared to the Helgoland selfing, including reduced expression of transcripts related to cellular stress responses. This implies that both intraspecific crosses maintained a growth response similar to the Helgoland selfing with reduced metabolic regulation during sublethal heat stress, indicating subtle heterosis (hybrid vigour) as a beneficial effect of outbreeding. Results are discussed in the frame of mariculture and marine forest restoration.