Granzyme A (GzmA), a serine protease highly expressed in cytotoxic immune cells, plays complex roles in antitumor immunity and inflammation. While elevated GzmA levels correlate with favorable outcomes in certain cancers, extracellular GzmA has been implicated in promoting tumorigenesis via inflammatory pathways. These contrasting functions highlight the need for selective tools that can detect GzmA activity with high spatiotemporal resolution in native biological contexts. A series of quenched activity-based probes that fluoresce only upon covalent binding to active GzmA have been developed. The lead probe, featuring the Ile-Gly-Asn-Arg recognition sequence, a phenyl phosphinate ester warhead, and a near-infrared sulfo-Cy5/QSY21 fluorophore/quencher pair, exhibits high reactivity, selectivity, and cell permeability. It enables robust detection of GzmA in vitro and in cells, with an excellent signal-to-noise ratio. These activatable probes will support downstream activity-based protein profiling and enable noninvasive imaging of the enzyme activity, offering a powerful means for dissecting the multifaceted biology of GzmA within the tumor immune microenvironment.