We synthesized fluorescent Cd nanoclusters (CdNCs) through a protein-directed method, and the synthesis method was utilized for a homogeneous, ultrasensitive, and selective detection of cadmium ion (Cd(2+)). CdNCs were synthesized using a modified protein-directed method for developing a rapid Cd(2+) detection system. For rapid Cd(2+) detection, the reaction time was reduced by optimizing the reaction conditions such as temperature, reducing agent concentration, and protein concentration. The synthesized CdNCs had ca. 2 nm diameter and showed strong fluorescence at 485 nm under 365 nm UV light. The fluorescence of the CdNCs increased with increasing Cd(2+) concentrations, and the limit of detection in deionized water was 15.68 fM. This method enables the detection of Cd(2+) through the Cd concentration-dependent formation of fluorescent CdNCs in tap, fountain, and pond water samples with detection limits of 0.75, 7.65, and 48.2 fM, respectively. The sensitivity and specificity of our method are comparable to those of several existing methods for Cd(2+) detection. Furthermore, the system enables the homogeneous detection of Cd(2+) without separation and washing, thereby broadening its application in analytical chemistry.