By means of single-crystal X-ray diffraction, we give direct crystallographic evidence of a centrosymmetry breaking below T-s = 200 K, concomitant with the onset of a commensurate structural modulation in the quadruple perovskite YMn3Mn4O12. This result, which explains the anomalously large thermal coefficient of the Y3+ ion in previously reported structural models, is attributed to the small size of the Y3+ ion, which causes its underbonding within the dodecahedral coordination polyhedron. The present data are consistent with a commensurate superstructure described by an I-centered pseudo-orthorhombic cell with polar Ia symmetry and a approximate to a(F)root 2 = 10.4352(7) angstrom, b approximate to 2b(F) = 14.6049(9) angstrom, c approximate to c(F)root 2 = 10.6961(7) angstrom, and beta = 90.110(3)degrees, where a(F) approximate to c(F) approximate to 7.45 angstrom, b(F) approximate to 7.34 angstrom, and beta approximate to 91 degrees are the unit cell parameters of the I2/m structure observed at room temperature. Consistent with the above polar structure, at lower temperature, T* = 70 K, we observe in polycrystalline samples an anomaly of the direct current (DC) and alternating current (AC) magnetization, concomitant with the appearance of a net electric polarization, as indicated by pyrocurrent and dielectric constant measurements. These results, complemented by electrical transport measurements, suggest a magnetic ferroelectricity driven by short-range magnetic order in YMn3Mn4O12.