Provided is a separator for an electrochemical element having exceptional strength and shielding performance, a small thickness, and low resistance. A separator for an electrochemical element interposed between a pair of electrodes and capable of holding an electrolytic solution containing an electrolyte, wherein the separator for an electrochemical element comprises a regenerated cellulose fiber having an average fiber length of 0.25-0.80 mm and an average fiber width of 3-35 μm, and in which the value calculated by dividing the average fiber length by the average fiber width is 15-70.