<p>The presence of gases such as carbon dioxide (CO₂) and methane (CH₄) in shallow groundwater from sources, including geological sequestration, contaminant degradation, and energy development and storage, can reduce groundwater quality and cause greenhouse gas emissions. Therefore, it is important to have high-resolution methods to measure dissolved gases in the subsurface. Traditional methods of active sampling (e.g., collecting groundwater samples for dissolved-gas analysis) are labor-intensive and have high potential for error. A low-cost, 3D-printed probe to measure dissolved CO₂ was developed and tested in the laboratory. The probe was constructed using an air-filled 3D-printed housing containing a pressure and NDIR sensor. The housing was coated in a silicone membrane that isolated ambient water from the air-filled chamber within the probe, similar to a total dissolved gas pressure (TDGP) probe. This allows the partial pressure of CO₂ and the dissolved gas concentration in the ambient water to be measured. The probe concept and design were tested for accuracy by using the probe to measure dissolved CO₂ in water of known concentration. After calibration, it was found that the average measurement error in the tested concentration range was ± 2.3%. Further, two probe designs with different characteristics were tested to optimize the equilibration time and showed agreement with theoretical models. The novel low-cost probe provides a valuable tool to monitor dissolved CO₂ in groundwater systems for a variety of applications and can increase the availability of data collected at sites where gases are of interest or concern.</p>