In this work, we demonstrated the development of a colorimetric immunosensor using surface plasmon resonance band of gold nanoparticles for the detection of prostate specific antigen (PSA). To develop this biosensing tool, triangular gold nanoparticles (AuNPs) were synthesized using Tween-20 as a nonionic surfactant and then, conjugated with PSA capture antibody (Ab1-AuNPs). When exposed to Ab1-AuNPs, PSA antigens were found to be successfully captured by nanosystem (PSA)-Ab1-AuNPs. Next, (PSA)-Ab1-AuNPs were incubated with second PSA antibody (2)-decorated magnetite (Fe3O4-Ab2) and separated by an external magnetic force to leave Ab1-AuNPs in the supernatant solution to be directly analyzed using UV-Vis spectroscopy. It was found that the absorption intensity was directly proportional to the PSA concentration. As a result, the linear range for PSA detection was found to be 0.01-20 ng mL-1 with a detection limit of 0.009 ng mL-1. Because of significant stability of the prepared Ab1-AuNPs and excellent selectivity to the PSA antigen, this simple and sensitive sensing system is proposed to be potentially effective in the fast and real-time analysis of clinical samples from prostate cancer patients. We believe that the simple platform of this immunosensor to be useful in the development of future point-of-care sensing tools, working on the quantification of biomarkers in a drop of blood.