Relay cropping is a multiple cropping system that consists in planting a second crop (i.e. relay crop) into a standing first crop (i.e. primary crop) prior to the first crop harvest allowing two potential harvests per year. Relay cropping has potential to address food security and environmental sustainability via spatio-temporal diversification of cropping systems. This is especially the case for soybean that presents key characteristics required for a relay crop including the market availability of a wide panel of genotypes, capacity to resist to and recover following mechanical damages via ramification, good combination as a relay crop of small-grain cereals etc. Despite these potential advantages, soybean as relay crop is poorly adopted worldwide, especially in Europe. For instance, in France and Europe, there has been a steady increase in soybean acreage in the last years under conventional cropping (i.e. crop grown in pure stand with traditional sowing date) while no official record exist to date on soybean grown by farmers under relay cropping. This lack of adoption of the crop under relay cropping could be due to several technical locks characterizing relay cropping system in general. A better understanding of the factors affecting adoption of soybean under relay cropping is therefore a prerequisite to put in place key mitigation strategies. In light of this, and based on a review of grey and scientific literature as well as the authors’ practical experience, here we report key knowledge gaps affecting the adoption of soybean under relay cropping with a particular emphasis on crop management complexities. Finally, we propose three research priorities to fill the current knowledge gaps in soybean relay cropping research and implementation, worldwide.