Context : Hydrogen peroxide ( HOOH ) was recently detected toward \rho Oph A . Subsequent astrochemical modeling that included reactions in the gas phase and on the surface of dust grains was able to explain the observed abundance , and highlighted the importance of grain chemistry in the formation of HOOH as an intermediate product in water formation . This study also predicted that the hydroperoxyl radical HO _ { 2 } , the precursor of HOOH , should be detectable . Aims : We aim at detecting the hydroperoxyl radical HO _ { 2 } in \rho Oph A . Methods : We used the IRAM 30m and the APEX telescopes to target the brightest HO _ { 2 } lines at about 130 and 260 GHz . Results : We detect five lines of HO _ { 2 } ( comprising seven individual molecular transitions ) . The fractional abundance of HO _ { 2 } is found to be about 10 ^ { -10 } , a value similar to the abundance of HOOH . This observational result is consistent with the prediction of the above mentioned astrochemical model , and thereby validates our current understanding of the water formation on dust grains . Conclusions : This detection , anticipated by a sophisticated gas-grain chemical model , demonstrates that models of grain chemistry have improved tremendously and that grain surface reactions now form a crucial part of the overall astrochemical network .