The microalgal bioactive components are revolutionizing medical diagnosis and therapy. Phycobiliproteins (PBP) of microalgae, viz., cyanobacterial phycoerythrin (C-PE), phycocyanin (C-PC), and allophycocyanin (C-APC), are the light-harvesting fluorescent proteins of the auxiliary photosynthetic complex of phycobilisomes (PBS). As part of medical therapy, PBPs support antioxidant, antibacterial, antifungal, antiviral, anti-corona, anti-inflammatory, anticancer, hepatoprotective, nephroprotective, neuroprotective, anti-amyloidogenic activity, anti-atherosclerosis, anti-aging, hypoglycemic, antihypertensive, immunomodulatory, wound healing, and anti-melanogenic. PBPs are used as photosensitizers in photodynamic therapy (PDT) for cancer. Fluorescent properties of PBPs, like excitation/emission at specific wavelengths, high fluorescence quantum yields, and large Stokes shifts, make PBPs a better fluorescent protein tag. Further ability of PBPs to form conjugates with many antibodies, ability to form tandem conjugates, PE-APC conjugates, use linkers for conjugate, label DNA probes, ability to attach to magnetic beads and array plates through bioconjugation are exploited for diagnostic applications. Diagnostic techniques such as liquid biopsy for cancer detection, flow cytometry, ELISA, Western blots, and microarray technology use PBPs to detect cells, proteins, DNA, antigens, and other biomolecules. Additionally, PBPs can be used to develop targeted drug delivery systems, as they bind to specific molecules. The purpose of this chapter is to explain the application of PBPs as diagnostic and therapeutic tools in human medicine.