We show, for the first time, that polarization-entangled photon pairs can be produced from the biexciton radiative cascade in a semiconductor quantum dot. We select an energy window that erases the ``which path'' information contained in the colors of the emitted photons, and use tomographic analysis to demonstrate that the photon pair violate Bell's inequality and satisfy Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty. Our quantitative analysis show that semiconductor quantum dots can be used as sources for ``event-ready'' entangled photons.