A new method, based on a renewable-reagent fiber optic sensor, for measuring the partial pressure of CO2 (pCO2) In seawater Is presented. The sensor operates by measuring the light Intensity at the absorbing wavelengths of a colorimetric acid-base Indicator which Is continuously delivered to the fiber tips through capillary tubing. The light Intensity Is modulated by pH changes that occur when CO2 diffuses across a gas-permeable membrane. The sensor operates both In a diffusion-dependent steady state and equilibrium regime depending upon the Indicator flow rate. At low flow rates, an equilibrium model can be used to predict the response of the sensor. The results Indicate that the sensor operates within the steady-state regime at flow rates higher than approximately 0.2 ML/min. The optimal precision Is ±0.8 gatm from 300-550 gatm of CO2, calculated from the response sensitivity and 3⨯ the root mean square noise. Response times (100 %) range from 11 to 26 min and depend upon the Indicator flow rate. Sensitivity to temperature and sample hydrodynamics Is also discussed. The sensor performance was tested on a research cruise, and these results are compared to the underway pCO2 measured simultaneously by an Infrared CO2 analyzer.