A new plate fluorosensor foil was developed for two-dimensional pH measurement in marine sediments and overlying waters. The fluorescent dye 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) was covalently linked onto a transparent poly(vinyl alcohol) membrane backed by polyester sheet. Both excitation and emission bands of the immobilized HPTS showed large red shifts in comparison to HPTS in free solution. The ratio of fluorescence emission intensities at 540 nm following successive excitation at 506 and 428 nm was pH-sensitive and correlated with pH changes from 5.5 to 8.6. These properties are robust and insensitive to factors such as intensity of light source, temperature, and oxygen concentration. The plate sensor responded rapidly (<2 min), reversibly, and with high precision to pH changes in solution and in sediment. The stability of the sensor was also tested by continually changing the pH between 6 and 8. Performance of the sensor did not vary after >200 pH cycles. Furthermore, the sensor foil retained its original properties after continuous exposure to natural marine sediment for 2 months. The plate optode has been successfully used to measure two-dimensional pH distributions in intertidal flat and subtidal sediment. Images are readily obtained with inexpensive light-emitting diode (LED) excitation and commercial-grade digital cameras with typical pixel resolution of ∼ 50 × 50 μm over areas > 150 cm².