Understanding the mechanisms guiding the early marine migration of Atlantic salmon (Salmo salar) post-smolts is essential for improving conservation and management strategies amid declining marine survival. We combined acoustic telemetry, high-resolution fjord oceanography, and individual-based biophysical modeling (IBBM) to investigate post-smolt navigation and progression through the Hardangerfjord system, southwestern Norway. Tagged salmon from two rivers were tracked as they migrated toward the open sea, and their fjord residence time, exit success, and exit route were compared with model simulations representing seven behavioral scenarios ranging from passive drift to active, goal-oriented swimming. Telemetry revealed an apparent migration success of 37–38% for both rivers, fjord residence times averaging 3–5 days (range 1–14 days), and mean progression speeds of 0.9–1.5 body lengths s⁻¹. Migration routes varied notably among individuals from both rivers, indicating diverse movement strategies within the fjord. Although simplified simulations incorporating passive drift or single environmental cues captured some aspects of post-smolt movement, models combining active directional swimming with hydrodynamic forcing —particularly current advection—best reproduced empirical data, suggesting that post-smolts integrate active orientation with environmental forcing to achieve efficient seaward migration. These findings support the idea that smolts use both local environmental cues and a broader compass mechanism while navigating through fjord environments, jointly supporting orientation toward the open sea. This integrative approach advances the understanding of how behavior and environmental forcing shape migration dynamics, offering new insights for spatially informed management of migratory salmon populations.