A marine riser tensioner is a device used on an offshore drilling vessel which provides a near constant upward force on the drilling riser independent of the movement of the floating drill vessel.

The marine riser is connected to the wellhead on the sea bed and therefore the tensioner must manage the differential movements between the riser and the rig. If there were no tensioner and the rig moves downward, the riser would buckle; if the rig rises then high forces would be transmitted to the riser and it would stretch and be damaged.

Tensioners have historically been composed of hydraulic actuated cylinders with wire sheaves. More recently, active electrical motors have been used for compensation purposes.