Herein, we present the use of a single gold nanorod sensor for detection of diseases on an antibodyfunctionalized surface, based on antibody–antigen interaction and the localized surface plasmon resonance (LSPR) lmax shifts of the resonant Rayleigh light scattering spectra. By replacing the cetyltrimethylammonium bromide (CTAB), a tightly packed self-assembled monolayer of HS (CH2)11(OCH2CH2)6OCH2COOH(OEG6) has been successfully formed on the gold nanorod surface prior to the LSPR sensing, leading to the successful fabrication of individual gold nanorod immunosensors. Using prostate specific antigen (PSA) as a protein biomarker, the lowest concentration experimentally detected was as low as 111 aM, corresponding to a 2.79 nm LSPR lmax shift. These results indicate that the detection platform is very sensitive and outperforms detection limits of commercial tests for PSA so far. Correlatively, its detection limit can be equally compared to the assays based on DNA biobarcodes. This study shows that a gold nanorod has been used as a single nanobiosensor to detect antigens for the first time; and the detection method based on the resonant Rayleigh scattering spectrum of individual gold nanorods enables a simple, label-free detection with ultrahigh sensitivity.