Magnetically driven ferroelectric atomic displacements of the order of 10-3 Å have been observed in orthorhombic (perovskite like) YMnO3 by a single-crystal synchrotron x-ray diffraction. The refined polar structure shows the characteristic bond alternation driven by the exchange striction in staggered Mn-O-Mn arrays with ↑↑↓↓ type ordering, giving rise to a spontaneous polarization along the a axis. First-principles calculations based on the Berry phase method as well as on the experimentally refined crystal structure can reproduce the observed polarization value.