A colorimetric sensor is proposed for the determination of cobalt. It is prepared by immobilizing 1-(2-pyridylazo)-2-naphthol (PAN) in an optically transparent polymethacrylate matrix. The conditions for determining cobalt by the proposed colorimetric sensor are found in the study of the complexation of cobalt with a PAN-modified polymethacrylate matrix. The determination is based on the interaction of the sensor with a cobalt solution at pH 4 for 5 min, followed by the measurement of absorbance at 620 nm. The sensor ensures the determination of 0.05–0.50 mg/L of cobalt with the detection limit 0.03 mg/L. It is demonstrated that the sensor can be used for the determination of cobalt in the Cyanocobalamin medical preparation and potable water.