Significance Magnetoresistance is the change of resistance in the presence of an external magnetic field. In rare-earth manganite compounds, this change is orders of magnitude stronger than usual and it is promising for developing new spintronic and electronic devices. The colossal magnetoresistance (CMR) effect has been observed only in chemically doped manganite compounds. We report the realization of CMR in a compressed single-valent LaMnO 3 manganite compound. Pressure generates an inhomogeneous phase constituted by two components: a nonconductive one with a unique structural distortion and a metallic one without distortion. The CMR takes place when the competition between the two phases is at a maximum. We identify phase separation as the driving force for generating CMR in LaMnO 3 .