An expanding disk gate valve assembly comprises two gate disks and an elastomeric disk. A first flange on a first gate disk mates to a second flange on a second gate disk forming an internal volume surrounding the elastomeric disk. An elastomeric sheath covers the mated disks with the first gate disk being radially offset from, and radially movable relative to, the second gate disk. A width of the gate disk assembly is less than a width between two valve seats when the gate disk assembly is in an open position. In a closed position, the first flange moves radially toward the second flange, compressing elastomeric disk, and causes the elastomeric disk to expand axially against the two gate disks, forcing them apart, and increasing the gate disk assembly width so that the gate disk assembly actively seals against the valve seats.