Biodegradable plastics are increasingly replacing conventional polymers, yet their environmental fate and ecotoxicological impacts in marine systems remain poorly defined, particularly for poly(butylene adipate‑co‑terephthalate) (PBAT), one of the fastest‑growing biodegradable polyesters. Here we present an integrated framework tracing PBAT‑derived dissolved organic matter (DOM) from its chemical identity through its physiological impacts on marine primary producers to its community‑level ecological consequences in coastal waters. Using kinetic leaching experiments coupled with ultrahigh‑resolution mass spectrometry and optical spectroscopy, we found that PBAT exhibited substantially higher DOM leaching rates in seawater than conventional or other biodegradable plastics. Leaching accelerated with elevated temperature, salinity, and UV radiation but was suppressed by surface biofilms. Molecular analysis revealed that PBAT‑derived DOM was enriched in aromatic and aliphatic CHO‑type compounds, distinct from natural organic matter. Laboratory assays with two diatom species showed dose‑dependent physiological stress, characterized by impaired photosynthetic efficiency, elevated reactive oxygen species, and compromised membrane integrity. Field dilution experiments further demonstrated that PBAT leachates significantly suppressed micro‑phytoplankton (20–200 μm) growth, whereas pico‑phytoplankton (<2 μm) remained resilient. These findings reveal that rapid DOM leaching from PBAT exerts size‑structured, community‑level effects on marine phytoplankton. Collectively, our results demonstrate that the molecular design of "biodegradable" polymers dictates their ecotoxicological impact, urging a shift from persistence‑only risk assessment toward one that includes leachate‑driven toxicity.