Easy-to-use and sensitive quantification of biomarkers has a great significance in disease prediction, diagnosis, and monitoring. Here, we report a biosensor for simple and sensitive biomarker detection based on the strong light scattering (brightness) of superradiative plasmonic nanoantennas. This nanoantenna is constructed using antibody-decorated gold nanoparticles (Au NPs) immobilized onto a gold mirror by the target antigen, forming a nanoparticle-on-mirror (NPOM) configuration. The NPOM produces an order of magnitude stronger light scattering in the red region compared with isolated Au NPs on the dielectric substrate, due to the strong near-field coupling of surface plasmons across the gap between the Au NPs and the gold film. The increased brightness allows one to observe the captured Au NPs with the naked eye using a dark-field optical microscope. The particle density of the Au NPs varies linearly with the concentration of the target antigen over a broad dynamic range from 10⁻³ to 10³ ng mL⁻¹. This dynamic range is three orders of magnitude broader than that obtained from the previous work based on a dark-field optical microscope. The limit of detection is 1 pg mL⁻¹ (6.67 fM), which is three orders of magnitude more sensitive than that obtained in the previous work using similar conditions. The uniform spatial distribution of the Au NPs on the gold film was allowed to quantify biomarkers with a relative standard deviation as small as 1–7%. Biosensing using superradiative NPs can lower the detection limit, simplify, and speed up the detection procedure for biomarker detection.