Abstract: In this study, we develop a harmonized emission factor-based assessment framework, coupled with established emission models, to quantify and compare GHG emissions from Atlantic salmon and rainbow trout aquaculture in Norway, Chile, the United Kingdom, Canada, and China over the period 2003—2023, expressed in carbon dioxide equivalents. We analyze temporal dynamics, cross-country contrasts, and spatial aggregation characteristics of emissions. Across the five countries, total salmonid production increased from 1.27 to 2.71 million tons (114%), whereas aggregate GHG emissions grew at a markedly slower pace (85%). Notably, nitrous oxide (N₂O) emissions declined by 36.8%, underscoring the significant role of improved feed efficiency and advances in farming technology in achieving emission reductions. At the species level, rainbow trout consistently exhibited lower emission intensities than Atlantic salmon. National dynamics diverged substantially: Norway showed a sustained reduction in emission intensity driven by efficiency gains; Chile displayed interannual variability associated with marine environmental conditions; the United Kingdom and Canada maintained relatively stable emission profiles; and China achieved progressive intensity reductions through optimization of production systems and technological upgrading. Spatially analysis further revealed significant clustering of salmonid-related GHG emissions, with hotspots concentrated along Norway’s western coast, southern Chile (Los Lagos Region and Aysén Region), north-west Scotland, British Columbia, and the north-western plateau regions of China, underscoring the close coupling between farming intensity and regional ecological carrying capacity. Collectively, these findings elucidate the evolving spatiotemporal structure of greenhouse gas emissions in global salmonid aquaculture and demonstrate a partial decoupling of emission growth from production expansion. This work provides a robust quantitative basis for advancing low-carbon development pathways and designing region-specific mitigation strategies for cold-water salmonid aquaculture.