A low-cost on-paper sensor based on 5,10,15-tritolylcorrolatocobalt(III) triphenylphosphine, CoTTCorr(PPh₃), was developed for cyanide detection in aqueous solutions. The sensor was coupled to a smartphone and used a home-written color intensity analysis software in order to record and interpret the colorimetric response. The detection of cyanide was possible down to 0.053 mg/L, an order of magnitude lower than the value of 0.5 mg/L set by the World Health Organization (WHO) for safe short-term exposure of cyanide in potable water. The colorimetric sensor had selectivity toward cyanide ions over the anions Cl-, Br, F-, NO₂, SCN-, OA[Formula: see text]-,ClO₄-, H₂PO₄- and HCO₃- while the influence of NO₃- ions on the sensor optical response towards cyanide was overcome by optimization of the ionophore/anion-exchanger ratio inside the sensing material. The best performance was obtained for the optode with an ionophore to exchanger ratio of 1:3. The optimized optodes were employed for quantification of cyanide content added to potable water and saliva.