Needle-free jet injection is a transdermal drug delivery technique wherein a liquid drug is pressurized, and ejected through a ~200 μm orifice at high speed (~200 m/s). The resulting fluid jet can rapidly penetrate through the skin, and disperse in the underlying tissue at a speed-related depth. Our electronically controllable injection systems uniquely offer the possibility of depth-control during injection. To this end, we have developed a spatially-resolved diffuse imaging technique to provide an estimate of the injection depth. An injection system was constructed to couple a collimated laser beam into the fluid jet as it was ejected through the orifice. During an injection, the penetration of the jet into a tissue-mimicking phantom eroded an unobstructed optical path for the laser beam before it impinged on the scattering medium at the bottom of the hole. This resulted in a pattern of backscattered light around the injection site that varied as a function of injection depth. We performed laser-coupled injections into a light-scattering polyacrylamide gel, while recording high-speed videos of the diffuse light exiting from the side and surface of the phantom. The centroid of the light distribution exiting from the side of the phantom was used as the estimate for the injection depth. A strong correlation was found between the depth of the centroid and the surface light profile, showing that it is possible to infer the injection depth from the spatial distribution of light around the injection site alone.