Biological nitrogen fixation (BNF) is a significant source of N in growing and filling grain legumes, but its contribution to the seed N economy and its possible links to seed longevity, germination ability, and vigour remain poorly understood. This narrative review discusses how reproductive-phase BNF affects seed nitrogen supply, remobilisation, N assimilation, storage protein accumulation, maturation, and seed quality. Isotope-based nitrogen partitioning studies show that reproductive BNF can contribute a significant amount of N to the N economy of developing legume seeds, and that vegetative N remobilisation is an important additional source. Nitrogen availability may influence seed development through the GS/GOGAT pathway for nitrogen assimilation and subsequent incorporation into storage proteins. Concurrently, ABA-induced maturation, LEA proteins, raffinose family oligosaccharides, redox balance and genome-protective mechanisms are known factors linked to seed desiccation tolerance, emergence and longevity. Direct evidence linking variation in reproductive BNF to these downstream seed-protection systems and mature-seed vigour remains limited. Therefore, we suggest a mechanistic model in which reproductive BNF affects the seed nitrogen pool and influences developmental and environmental cues during seed filling, and we propose that the BNF-to-seed-protection relationship is a major area for future research. This framework needs to be tested in future studies using multiple analyses of seed vigour and longevity, combined with molecular profiling, nitrogen partitioning and isotope tracing, across con-trasting BNF environments. This synthesis provides a framework for considering reproductive BNF as a potential component of seed quality and seedling establishment, with implications for future crop improvement strategies.