The Red Sea and Arabian Gulf sustain a salinity-driven overturning circulation that links arid marginal seas to the northwestern Indian Ocean, yet its three-dimensional pathway connectivity remains incompletely resolved. Using 3D Lagrangian particle tracking within a well-validated eddy-resolving ocean model, we reconstruct this overturning framework in full and reveal a fundamental vertical partitioning of the outflow. The framework separates into a rapid, transient surface path and a dominant intermediate-depth (100–800 m) path with multi-year travel time. This subsurface branch carries the majority of the salinity anomaly and constitutes the primary reservoir through which marginal-sea buoyancy forcing is communicated to the broader Arabian Sea. The inflow branches are traced to distinct upstream source regions along the southern Somali margin and central Arabian Sea. These findings establish a pathway-based salinity-driven overturning framework for understanding how marginal seas influence intermediate hydrography, ventilation, and the broader Indian Ocean circulation.