A turbine system is disclosed. In one embodiment, the turbine system includes a transition duct. The transition duct includes an inlet, an outlet, and a passage extending between the inlet and the outlet and defining a longitudinal axis, a radial axis, and a tangential axis. The outlet of the transition duct is offset from the inlet along the longitudinal axis and the tangential axis. The transition duct further includes an interface feature for interfacing with an adjacent transition duct. The turbine system further includes a flexible seal contacting the interface feature to provide a seal between the interface feature and the adjacent transition duct. The flexible seal includes a sheet having a first surface, an opposing second surface, and a peripheral edge therebetween.