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Convenient labeling of proteins is important for observing its function under physiological conditions. In tissues particularly, heptamethine cyanine dyes (Cy-7) are valuable because they absorb in the near-infrared (NIR) region (750&#8315;900 nm) where light penetration is maximal. In this work, we found Cy-7 dyes with a <i>meso</i>-Cl functionality covalently binding to proteins with free Cys residues under physiological conditions (aqueous environments, at near neutral pH, and 37 &#176;C). It transpired that the <i>meso</i>-Cl of the dye was displaced by free thiols in protein, while nucleophilic side-chains from amino acids like Tyr, Lys, and Ser did not react. This finding shows a new possibility for convenient and selective labeling of proteins with NIR fluorescent probes.