A stent graft having an internal bidirectional branch formed from a tubular segment of graft material. The internal bidirectional branch extends within the lumen of the stent graft and proximally and distally from a lateral opening in the sidewall of the stent graft. The tubular segment from which the stent graft is made is partitioned into first and second sections along a length of the tubular segment to form the internal bidirectional branch. The lateral opening has a length and a width that may be greater than the diameter of the internal bidirectional branch and may be in the shape of a quadrilateral. The internal bidirectional branch and the stent graft are formed from a single piece of graft material.