A rapid, simple, and reliable competitive immunoassay was developed and validated for measurement of Cd(II) in environmental water samples. This assay employed a monoclonal antibody that recognizes Cd(II)−EDTA complexes as capture reagent and a Cd(II)−EDTA conjugate of horseradish peroxidase as an enzyme label. The assay depended on a competitive binding reaction between the enzyme conjugate and Cd(II)−EDTA complexes, derived from the environmental water sample, for the binding sites of the immobilized antibody. The concentration of Cd(II) in the sample was quantified by the ability of its EDTA complexes to inhibit the binding of the enzyme conjugate to the antibody and, subsequently, color formation in the assay. The assay was specific to Cd(II), with a limit of detection of 0.3 ppb. Ca(II), Mg(II), and Fe(III), the metal ions commonly found in ambient water at relatively high concentrations, did not interfere with the assay. Mean analytical recovery of added Cd(II) was 100.29 ± 3.60. The precision of the assay was satisfactory; coefficients of variation were 3.6−10.9 and 4.81−10.21% for intra- and interassay precision, respectively. The assay compared favorably with graphite furnace atomic absorption spectroscopy in its ability to accurately measure Cd(II) spiked into water samples from a Louisiana bayou.