Abstract: The invention relates to wind and hydraulic power engineering and can be used at tidal power plants, low-head river hydroelectric plants, wave power plants, and wind power plants to increase turbine efficiency by further reducing the relative power of idle jets in the flow chamber of an orthogonal turbine. The turbine comprises a rotor with wing-shaped profile blades mounted transversely in the flow chamber. The chamber has a transverse protrusion the upper face of which borders, with clearance, the surface of the cylinder defined by blades. In the cross-section perpendicular to the axis of the rotor, the side face of the transverse protrusion facing the inlet opening of the flow chamber is made concave, and tangent to this face forms an acute angle, with a straight line segment connecting the tangency point with the axis of the rotor, in the direction of the inlet opening of the flow chamber.