The superionic electrical conductor Ag4SSe is reported to undergo an unusual first-order structural phase transition at ∼260K with concomitant anomalous electronic properties. Single-crystal x-ray diffraction reveals that the crystal undergoes a structural transition from monoclinic α-Ag4SSe to an orthorhombic crystal structure below 260 K. The new lowerature phase is denoted as δ-Ag4SSe, and it is found to be isostructural to α-Ag2Se. The diamagnetic magnetic susceptibility is of larger magnitude in the lowerature δ phase than in α-Ag4SSe. A diamagnetic susceptibility of larger magnitude is usually related to a lower density of states at the Fermi level which leads to a decrease in the Pauli paramagnetic susceptibility. However, we find δ-Ag4SSe to be metallic with a much lower resistivity than semiconducting α-Ag4SSe. As a consequence, the α-δ structural phase transition of Ag4SSe causes an unusual Fermi surface reconstruction with an increased mobility of charge carriers and unexpected metallic behavior in δ-Ag4SSe. Band structure calculations substantiate such a claim. In addition, we also observe spontaneous voltage generation (SVG) at the phase transition, which has previously been observed only in a few magnetic materials.