A method for determining the shape and position of an underwater riser extending from a floating platform includes calculating a deformed shape of the riser by means of a numerical model of the deformed shape of the riser as a function of a plurality of acceleration values and of a plurality of position values in predetermined points of the riser, detecting acceleration values of the riser in a plurality of detection points along a longitudinal extension of the riser, detecting the water pressure values in at least some of the detection points, and calculating the position values as a function of the measured water pressure values.