Respiratory viruses, such as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), influenza A (H1N1), and respiratory syncytial virus (RSV), remain major public health threats due to overlapping clinical symptoms and the limited speed, sensitivity, and multiplexing capacity of conventional diagnostic methods. Here, we present a label-free optical biosensing platform based on subwavelength grating microring resonators (SGMRRs) for the sensitive and specific detection of viral antigens. The sensor surface is functionalized with antibodies against three representative viral proteins, enabling quantitative detection via resonance wavelength shifts upon antigen binding. The platform achieves the refractive index limit of detection (LoD) of 1.25×10⁻⁴ RIU and enables the detection of the SARS-CoV-2 spike protein at 100 pg/mL, the H1N1 nucleoprotein at 10 pg/mL, and the RSV F-trimer protein at 10 pg/mL. The compact SGMRR design readily supports multiplexed integration through multi-ring or multi-channel architectures, highlighting its potential as a scalable and high-performance platform for rapid, quantitative, and point-of-care viral diagnostics.