Analysis of CCD photometric observations of the near-Earth asteroid (399307) 1991 RJ2 made in 2014 August-September show it to be a binary system with a primary period of 3.4907 ± 0.0002 h and orbital period of 15.917 ± 0.001 h. The depth of the secondary mutual event indicates a minimum effective diameter ratio (Ds/Dp) of 0.47 ± 0.02. The lightcurve showing the orbital period and mutual events indicates a slightly elongated (a/b ~ 1.07/1.0) satellite that is tidally-locked to the orbital period.