We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb2Fe4Se5 superconductor (T-c = 32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of (h) over bar omega(res) = 14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the root 5 X root 5 magnetic superstructure. The estimated ratios of (h) over bar omega(res)/k(B)T(c) approximate to 5.1 +/- 0.4 and (h) over bar omega(res)/2 Delta approximate to 0.7 +/- 0.1, where Delta is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111 and 122 pnictides.