We aim at unveiling the observational imprint of physical mechanisms that govern planetary formation in young , multiple systems . In particular , we investigate the impact of tidal truncation on the inner circumstellar disks . We observed the emblematic system GG Tau at high-angular resolution : a hierarchical quadruple system composed of low-mass T Tauri binary stars surrounded by a well-studied , massive circumbinary disk in Keplerian rotation . We used the near-IR 4-telescope combiner PIONIER on the VLTI and sparse-aperture-masking techniques on VLT/NaCo to probe this proto-planetary system at sub-au scales . We report the discovery of a significant closure-phase signal in H and K _ { s } bands that can be reproduced with an additional low-mass companion orbiting GG Tau Ab , at a ( projected ) separation \rho = 31.7 \pm 0.2 mas ( 4.4 au ) and PA = 219.6 \pm 0.3 ^ { \circ } . This finding offers a simple explanation for several key questions in this system , including the missing-stellar-mass problem and the asymmetry of continuum emission from the inner dust disks observed at millimeter wavelengths . Composed of now five co-eval stars with 0.02 \leq M _ { \star } \leq 0.7 M _ { \odot } , the quintuple system GG Tau has become an ideal test case to constrain stellar evolution models at young ages ( few 10 ^ { 6 } yr ) .