Gastrin-releasing peptide receptor (GRPr) plays proliferative and inflammatory roles in living systems. Here, we report a highly selective GRPr antagonist (JMV594)-tethered iridium(III) complex for probing GRPr in living cancer cells and immune cells. This probe exhibited desirable photophysical properties and also displayed negligible cytotoxicity, overcoming the inherent toxicity of the iridium(III) complex. Its long emission lifetime enabled its luminescence signal to be readily distinguished from the interfering fluorescence of organic dyes by using a time-resolved technique. This probe selectively visualized living cancer cells via specific binding to GRPr, while it also modulated the function of GRPr on TNF-α secretion in immune cells. To our knowledge, this is the first peptide-conjugated iridium(III) complex developed as a GRPr bioimaging probe and modulator of GRPr activity. This theranostic agent shows great potential at unmasking the diverse roles of GRPr in living systems.