Coastal phosphorus (P) burial represents a critical biogeochemical process regulating marine productivity and global P cycle, yet the mechanisms controlling its long-term sequestration during the Holocene remain poorly constrained. This study employs a multi-proxy approach (P speciation, AMS ¹⁴C dating, and XRF core scanning) to reconstruct P burial dynamics in the Southern Yellow Sea. Four depositional phases are identified, each controlled by sea-level rises and climatic forcing, with a critical transition at ~7.5 cal ka BP. This transition marks a shift from terrestrial-dominated to marine-driven P burial following sea-level stabilization. During 8.0-4.6 cal ka BP (DU3), P burial was dominated by terrestrial inputs linked to riverine discharge. Since 4.6 cal ka BP (DU4), marine productivity became the primary driver of P burial, with phytoplankton dominating sedimentary P sequestration (>80%). Mechanistic analyses reveal that sea-level rise amplified coastal erosion, simultaneously increasing terrestrial reactive P influx while reducing burial efficiency by ~30% through sediment reworking. Collectively, these findings quantitatively constrain sea-level-driven P transfer, highlight the critical role of phytoplankton-mediated burial during the mid- to late Holocene, and identify a tipping point in burial regimes at 7.5 cal ka BP. This study establishes a pre-anthropogenic baseline for coastal P cycling, critically informing models of modern eutrophication and ecosystem resilience under climate change.