For the first time a new kind of line defect with metallic characteristics has been discovered in optically transparent and insulating BaSnO3 perovskite thin films. It was identified using advanced atomic-resolution electron microscopy. The line defect has a very distinct atomic structure built up mainly from Sn and O atoms. When the electronic properties of the line defect were probed in the electron microscope, it was found to be metallic. Calculations based on theory from first principles confirm this unusual metallic property of the defect. This discovery demonstrates that extended line defects in crystals can accommodate remarkable properties and provide a unique way to engineer material properties at the sub-nanometer scale.