<p>The unique features of the Equatorial Undercurrent (EUC) and relations to the Indian Ocean Dipole (IOD) highlight the complexity and importance of the tropical eastern Indian Ocean (EIO), yet the EUC dynamics and associated physical-biogeochemical implications remain patchy. Here, mainly using <i>in situ</i> observations during April–May 2011, we provide direct evidence of the spring eastward EUC and its relations to the southward subsurface salty water and upwelling off Sumatra and reveal its biogeochemical responses. A strong eastward velocity of ~1.2 m/s at a depth of 120 m near the equator along the meridional section of 90°E clearly indicates the EUC. The continuum of subsurface salty water from the equator to the offshore region off Sumatra generally shows the pathway and dynamic bridge role of the EUC. The southward shoaling of isotherms and isohalines near the upper boundary of thermocline in the region off Sumatra implies the occurrence of weak upwelling from the subsurface salty water; the early onset of southeasterly wind associated with the positive IOD might be responsible for this phenomenon. The EUC is important in driving the spatial variability of the oxygen minimum zone (OMZ) and subsurface chlorophyll a maximum (SCM) in the tropical EIO. In particular, the EUC may act as a source of O₂, depressing the upward limit of the OMZ at the equator. Moreover, the eastward depressed EUC induces the downwelling of the OMZ and a deepened and weakened SCM from west to east along the equator. Influenced by EUC transport and upwelling, a southward extension of the OMZ with an uplifted oxycline occurred in the region off Sumatra, and a southward enhanced and shoaled SCM emerged. The results unraveled the dynamic linkages between the EUC and biogeochemical environments, constituting a considerable contribution to the understanding of the physical-biogeochemical-ecological interactions in the tropical EIO.</p>