GX 339-4 is a low mass X-ray binary (LMXB) that has been extensively studied since its discovery. All four X-ray states typically seen in X-ray binaries (XRBs) have been detected in this system, and X-ray observations of its relatively frequent outbursts have been very important in shaping the theory of LMXB outbursts. The relation between X-ray and radio emission from XRBs also relies heavily on observations of this source. However, absorption lines from the donor star in this system had never been detected because even in quiescence, the accretion disc dominates the optical spectrum. We have for the first time detected absorption lines from the donor star in near-infrared spectra of GX 339-4 obtained with X-shooter on the VLT. We confirm that the donor is a K-type subgiant and measure its radial velocity semi-amplitude to be 219 km/s, much lower than previously determined from the Bowen emission lines. This leads to a mass function of 1.91 ± 0.08 Msun, which means that GX 339-4 could harbor one of the lowest mass black holes known to date.