Aims: We performed an optical/infrared study of the counterpart of the low-mass X-ray binary KS 1731-260 to test its identification and obtain information about the donor. Methods: Optical and infrared images of the counterpart of KS 1731-260 were taken in two different epochs (2001 and 2007) after the source returned to quiescence in X-rays. We compared these observations with those obtained when KS 1731-260 was still active. Results: We confirm the identification of KS 1731-260 with the previously proposed counterpart and improve its position to α=17:34:13.46 and δ=-26:05:18.60. The H-band magnitude of this candidate showed a decline of ~1.7 mag from outburst to quiescence. In 2007 April we obtained R =22.8± 0.1 and I = 20.9 ± 0.1 for KS 1731-260. Similar optical brightness was measured in June 2001 and July 2007. The intrinsic optical color R-I is consistent with spectral types from F to G for the secondary although there is a large excess over that from the secondary at the infrared wavelengths. This may be due to emission from the cooler outer regions of the accretion disk. We cannot rule out a brown dwarf as a donor star, although it would require that the distance to the source is significantly lower than the 7 kpc reported by Muno et al. (2000, ApJ, 542, 1016). Chandra Fellow.