The ordered oxygen-deficient perovskite YBaMn2O5 has been synthesized as a monophasic sample. A variable temperature neutron diffraction study of this phase has been carried out. The crystal structure of this manganite was solved in the space group P4/nmm (a = 5.547 Angstrom and c = 7.649 Angstrom). This oxide, which is closely related to the YBaCuFeO5 structure, forms an alternate stacking of yttrium and barium layers along the c axis. It differs from the latter, however, by the existence of two distinct sites for manganese, corresponding to an ordering of the Mn3+ and Mn2+ cations, respectively. Elucidation of the magnetic structure of YBaMn2O5, from low temperature PND (powder neutron diffraction) data, leads to a G-type antiferromagnetic model; the superimposed Mn2+/Mn3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements,