Quantum dots (QDs) composed of a group I-IV-VI semiconductor, AgGeS, have been intensively investigated for constructing efficient energy conversion systems. However, their potential for photoluminescence (PL)-based applications has remained unexplored. Herein, the first successful preparation of AgGeS QDs exhibiting near-infrared (NIR) PL is reported. These AgGeS QDs with an average diameter of 4.2-4.6 nm has an almost constant energy gap at 1.48-1.45 eV, even when the Ge/(Ag+Ge) precursor ratio is varied from 0.05 to 0.90. A significant PL peak is observed at 920 nm, the intensity being enlarged with an increase in the Ge/(Ag+Ge) ratio. The use of AgGeS QDs prepared with Ge/(Ag+Ge) = 0.82 in the precursors result in a PL quantum yield (QY) of 11%, which is further enhanced to 40% through surface coating with a ZnS shell of 1.0 nm in thickness, with the PL peak wavelength being slightly blue-shifted to 900 nm. Following surface modification with 3-mercaptopropionic acid for homogeneous dispersion in aqueous solutions, the AgGeS@ZnS QDs are utilized as an NIR PL probe for in vivo bioimaging. PL signals are clearly detected from depths of at least 15 mm beneath the back skin of a mouse, demonstrating their deep-tissue imaging capability.