AbstractHere we explore the water transfer between the subtropical and tropical gyres of the South Atlantic Ocean to better understand its unique equatorward heat delivery. A Lagrangian technique is applied to the reanalysis product GLORYS2V4 in order to trace back the western boundary flow in the tropical (North Brazil Undercurrent, NBUC) and subtropical (Brazil Current) gyres. Most of the northward NBUC core transport (14.9 Sv at 8°S) arrives from the eastern boundary subtropical current (Benguela Current) via the zonal South Equatorial Current. This subtropical‐tropical transfer represents the core of the returning limb of the Atlantic meridional overturning circulation and accounts for most of the observed increase in heat and salt‐volume transports (0.18 PW and 0.19 Sv from 30°S to 8°S, respectively) across the South Atlantic. The NBUC also includes Antarctic Intermediate Water below 400 m (7.4 Sv at 8°S) coming from the interior subtropical gyre, as well as water from the current's surface and peripheral components coming from the tropical gyre (13.3 Sv at 8°S). The Brazil Current (9.9 Sv at 29°S) is mostly composed of subtropical water originating in the upper 800 m west of the eastern boundary current at 30°S (8.5 Sv), with a minor contribution of surface tropical water that transfers to the subtropics (1.4 Sv).