This work demonstrated the use of Au nanocages as a new class of lymph node tracers for noninvasive photoacoustic (PA) imaging of a sentinel lymph node (SLN). Current SLN mapping methods based on blue dye and/or nanometer-sized radioactive colloid injection are intraoperative due to the need for visual detection of the blue dye and low spatial resolution of Geiger counters in detecting radioactive colloids. Compared to the current methods, PA mapping based on Au nanocages shows a number of attractive features: noninvasiveness, strong optical absorption in the near-infrared region (for deep penetration), and the accumulation of Au nanocages with a higher concentration than the initial solution for the injection. In an animal model, these features allowed us to identify SLNs containing Au nanocages as deep as 33 mm below the skin surface with good contrast. Most importantly, compared to methylene blue Au nanocages can be easily bioconjugated with antibodies for targeting specific receptors, potentially eliminating the need for invasive axillary staging procedures in addition to providing noninvasive SLN mapping.