Me-Tetrazine-Cyanine5.5 is a near-infrared fluorescent probe and the methyl-tetrazine functionalized derivative of Cyanine5.5. Its methyl-tetrazine group reacts rapidly and with high selectivity with TCO-modified proteins, antibodies, or other biomolecules via inverse electron-demand Diels–Alder (IEDDA) chemistry, forming stable covalent conjugates without a catalyst. Emitting in the near-infrared window, this dye reduces biological autofluorescence and improves tissue imaging performance. Me-Tetrazine-Cyanine5.5 is widely used for bioorthogonal labeling, cellular and tissue imaging, protein tracking, and multicolor fluorescence analysis studies.