On the basis of ab initio calculation, we propose a new structure for the fundamental excitation of the reconstructed 30° partial dislocation in silicon. This soliton has a rare structure involving a fivefold-coordinated atom near the dislocation core. The unique electronic structure of this defect is consistent with the electron spin-resonance signature of the hitherto enigmatic thermally stable R centre of plastically deformed silicon. We present the first ab initio determination of the free energy of the soliton, which is also in agreement with the experimental observation. This identification suggests the possibility of an experimental determination of the density of solitons, a key defect in understanding the plastic flow of the material.