A tensioning device (15) for a riser (5) connecting a subsea borehole (7) with a floating installation (1) on the surface of the sea (11), where the tensioning device (15) is provided with telescoping tubes (27, 27') and also several evenly spaced hydraulic cylinders (31, 31') arranged in a peripherally encircling manner and mainly in the longitudinal direction of the riser, and where the tension in the riser is exerted through hydraulic pressure in said cylinder (31, 31'), the tensioning device (15) consisting of two successive, interconnected telescopic tensioning units (23, 25), the tensioning units (23, 25) being designed separately to maintain a prescribed tension in the riser (5).