Ongoing and predicted range loss of kelp forests in response to climatic stressors are pressing marine managers to look into the adaptive capacity of populations to inform conservation strategies. Characterising how adaptive genetic diversity and structure relate to present and future environmental variation represents an emerging approach to quantifying kelp vulnerability to environmental change and identifying which populations have the genetic material to handle future ocean conditions. The dominant Australian kelp, Ecklonia radiata, was genotyped from 10 locations spanning 2,000 km of coastline and a 9.5°C average temperature gradient, along the east coast of Australia, a global warming hotspot. ddRAD sequencing was used to generate 10,700 high quality single nucleotide polymorphisms (SNPs) and characterize levels of neutral and adaptive genomic diversity and structure. The adaptive dataset, reflecting portions of genome putatively under selection, was used to infer genomic vulnerability by 2050 under the RCP 8.5 scenario. There was strong neutral genetic differentiation between Australia mainland and Tasmania, but only weak genetic structure among mainland populations within the main path of the East Australian Current. Genetic diversity was highest in the centre of the range, and lowest in the warm-edge population. The adaptive SNP candidates revealed similar genetic structure patterns with a spread of adaptive loci across most warm (northern) populations. The lowest, but most unique, adaptive genetic diversity values were found in both warm and cool population edges, indicating local adaptation but low evolutionary potential. Critically, genomic vulnerability modelling identified high levels of vulnerability to future environmental conditions in Tasmania. Populations of kelp at range edges are unlikely to adapt and keep pace with predicted climate change. Ensuring the persistence of these kelp forests, by boosting resilience to climate change, may require active management strategies with assisted adaptation in warm-edge (northern) populations and assisted gene flow in cool-edge (Tasmania) populations.