The use of colorimetric bioassays for protein detection is one of the most interesting diagnostic approaches, but their relatively poor detection limits have been a critical issue. In this study, we developed an efficient colorimetric bioassay based on switchable linkers (SLs) for the detection of prostate-specific antigen (PSA), which is one of the most widely used protein biomarkers for the diagnosis of prostate and breast cancers. SLs can cross-link gold nanoparticles (AuNPs) to generate large-scale aggregates and thereby induce precipitation to achieve visual signal amplification. In addition, when SLs are occupied by target proteins (referred to as 'switch-off'), highly sensitive detection is enabled. To maximize sensitivity, we adjusted the total surface area of AuNPs by controlling their concentration. As a result, PSA was detected at an ultralow concentration of 100 fg mL-1. This SL-based assay is shown to be simple, easy to handle and visualize, and highly sensitive. Therefore, in addition to PSA, the proposed SL-based assay could be used to detect other protein biomarkers.