Fast and ultrasensitive detection of pathogens is very important for efficient monitoring and prevention of viral infections. Here, we demonstrate a label‐free optical detection approach that uses a printed nanochain assay for colorimetric quantitative testing of viruses. The antibody‐modified nanochains have high activity and specificity which can rapidly identify target viruses directly from biofluids in 15 min, as well as differentiate their subtypes. Arising from the resonance induced near‐field enhancement, the color of nanochains changes with the binding of viruses that are easily observed by a smartphone. We achieve the detection limit of 1 PFU μL−1 through optimizing the optical response of nanochains in visible region. Besides, it allows for real‐time response to virus concentrations ranging from 0 to 1.0×105 PFU mL−1. This low‐cost and portable platform is also applicable to rapid detection of other biomarkers, making it attractive for many clinical applications. A printed nanochain‐based colorimetric assay is utilized for point‐of‐care quantitative testing and real‐time monitoring of viral infection with high sensitivity, specificity, and reliability. It enables rapid detection of various pathogens from tiny amounts of clinical sample with a smartphone.