Recently, due to the corrosion problem in the petroleum industry and the usage of commercial corrosion inhibitors, which, when released untreated into the environment, are considered to be environmentally unfriendly, green corrosion inhibitors are being researched. In this paper, the results of dandelion-root-extract testing as a green corrosion inhibitor for carbon steel in simulated brine solution saturated with carbon dioxide (CO2) are shown. The extract’s inhibition efficiency in static and flow conditions was determined by using potentiodynamic polarization with Tafel extrapolation and electrochemical-impedance-spectroscopy methods. In static conditions, the extract was tested at different concentrations. A maximum inhibition efficiency of 98.37% in static conditions at an extract concentration of 12 mL/L and 82.80% in flow conditions at a concentration of 14 mL/L was achieved. Additionally, for the most efficient dandelion-root-extract concentration (12 mL/L), the biodegradability and toxicity were determined. A biodegradability of 0.96 and a toxicity of 2.38% was achieved. Based on the obtained results of the conducted laboratory measurements, it can be concluded that dandelion-root extract has significant potential in terms of its use as a green corrosion inhibitor for carbon steel in a CO2-saturated brine solution.