Robust marine carbon sensors with small size, low power consumption, and high sensitivity provide greater insight into the carbon cycle studies and resolve environmental variability. We report here the development of a diminutively integrated tunable diode laser absorption spectroscopy (TDLAS) system with a specially designed multipass gas cell for small amounts of dissolved gas extractions and measurements. It was used to detect and monitor carbon dioxide (CO2) dissolved in water and seawater. Systematic experiments have been carried out for system evaluation in the lab. Extracted CO2 was determined via its 4989.9 cm-1 optical absorption line. The achieved TDLAS measurement precision was 4.18 ppm for CO2, measured by averaging up to 88 s. The integrated absorbance was found to be linear to gas concentrations over a wide range. Comparison measurements of the atmospheric CO2 values with a commercial instrument confirmed a good accuracy of our TDLAS-based system. The first test campaign was also accomplished with a hollow fiber membrane contactor, and concentrations of CO2 were quantitatively detected with partial degasification operations. The results clearly show the ability to continuously measure dissolved gases and highlight the potential of the system to help us better understand physical and geochemical processes in a marine environment.