A CdSe/ZnS quantum dot (QD) based electrochemical immunoassay of phosphorylated bovine serum albumin (BSA-OP) as a protein biomarker is presented. The QDs were used as labels for amplifying electrochemical signals and were conjugated with a secondary anti-phosphoserine antibody in a heterogeneous sandwich immunoassay. In this assay, the model phosphorylated protein BSA-OP was added to the primary BSA antibody coated polystyrene microwells, and then the QD labeled anti-phosphoserine antibody was added for completing immunorecognition. Finally, the bound QD was dissolved in an acid-dissolution step and was detected by electrochemical stripping analysis. The measured current responses were proportional to the concentration of BSA-OP. Under optimal conditions, the voltammetric response was linear over the range of 0.5–500 ng mL⁻¹ of BSA-OP, with a detection limit of 0.5 ng mL⁻¹. It also shows good reproducibility with a relative standard deviation of 8.6%. This QD-based electrochemical immunoassay offers great promise for simple and cost-effective analysis of protein biomarkers.