The CoRoT satellite has provided high-precision photometric light curves for more than 163 000 stars and found several hundreds of transiting systems compatible with a planetary scenario. If ground-based velocimetric observations are the best way to identify the actual planets among many possible configurations of eclipsing binary systems, recent transit surveys have shown that it is not always within reach of the radial-velocity detection limits. In this paper, we present a transiting exoplanet candidate discovered by CoRoT whose nature cannot be established from ground-based observations, and where extensive analyses are used to validate the planet scenario. They are based on observing constraints from radial-velocity spectroscopy, adaptive optics imaging and the CoRoT transit shape, as well as from priors on stellar populations, planet and multiple stellar systems frequency. We use the fully Bayesian approach developed in the PASTIS (Planet Analysis and Small Transit Investigation Software) analysis software, and conclude that the planet scenario is at least 1400 times more probable than any other false-positive scenario. The primary star is a metallic solar-like dwarf, with Ms = 1.099 ± 0.049 M⊙ and Rs = 1.136^{+0.038}_{-0.090} R⊙. The validated planet has a radius of Rp = 4.88^{+0.17}_{-0.39} R⊕ and mass less than 49 M⊕. Its mean density is smaller than 2.56 g cm-3 and orbital period is 9.7566 ± 0.0012 d. This object, called CoRoT-22 b, adds to a large number of validated Kepler planets. These planets do not have a proper measurement of the mass but allow statistical characterization of exoplanets population.