Abstract Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are widely used as paleoclimate proxies on land, yet their origin in marine environments remains incompletely constrained. Here, we analyze suspended particulate matter collected from five depths at an open‐ocean station in the northern South China Sea and provide depth‐resolved evidence for substantial in situ brGDGT production within the oxic marine water column. BrGDGT concentrations show a pronounced subsurface maximum at ∼75 m and covary strongly with co‐occurring isoprenoid GDGTs, indicating that brGDGT synthesis is linked to marine microbial activity rather than being dominated by terrigenous supply alone. The brGDGT distributions are also distinct from those of terrestrial reference materials. These observations indicate that a significant fraction of marine brGDGTs can be produced well above the seafloor in oxic open‐ocean waters and they help constrain the ecological setting of the brGDGT signal ultimately exported to sediments.