Nanomaterials with luminescence in the short-wave infrared (SWIR) region are of special interest for biological research and medical diagnostics because of relatively favorable tissue transparency in that region. Single-walled carbon nanotubes (SWCNTs) show sharp SWIR spectral signatures and therefore have the potential for noninvasive detection and imaging of cancerous tumors when linked to selective targeting agents such as antibodies. However, such applications face the challenge of detecting and localizing the source of light emission from inside tissues. We present here a new method, called spectral triangulation, for sensitively locating SWCNT emission sources in three dimensions using only optical measurements at the specimen surface. SWCNTs inside tissue phantoms are excited by an LED array, and resulting SWIR emission is captured by a scanning fiber optic probe coupled to an InGaAs spectrometer or avalanche photodiode detector. Because of water absorption, attenuation of the SWCNT fluorescence in tissues is strongly wavelength-dependent. We can therefore gauge the distance between SWCNTs and probe by analyzing differential changes in the measured SWCNT emission spectra. SWCNT fluorescence is clearly detected though as much as 22.5 mm of tissue phantom, and the three-dimensional locations of SWCNT samples are deduced with sub-millimeter accuracy at depths up to 10 mm. Our method can also distinguish and locate dual SWCNT sources within tissues.