Phosphate oxyanions play central roles in biological, agricultural, industrial, and ecological processes. Their high hydration energies and dynamic properties present a number of critical challenges limiting the development of sensing technologies that are cost-effective, selective, sensitive, field-deployable, and which operate in real-time within complex aqueous environments. Here, a strategy that enables the fabrication of an electrolyte-gated organic field-effect transistor (EGOFET) is demonstrated, which overcomes these challenges and enables sensitive phosphate quantification in challenging aqueous environments such as seawater. The device channel comprises a composite layer incorporating a diketopyrrolopyrrole-based semiconducting polymer and a π-conjugated penta-t-butylpentacyanopentabenzo[25]annulene “cyanostar” receptor capable of oxyanion recognition and embodies a new concept, where the receptor synergistically enhances the stability and transport characteristics via doping. Upon exposure of the device to phosphate, a current reduction is observed, consistent with dedoping upon analyte binding. Sensing studies demonstrate ultrasensitive and selective phosphate detection within remarkably low limits of detection of 178 × 10−12 m (17.3 parts per trillion) in buffered samples and stable operation in seawater. This receptor-based doping strategy, in conjunction with the versatility of EGOFETs for miniaturization and monolithic integration, enables manifold opportunities in diagnostics, healthcare, and environmental monitoring.