A magnetostrictive torque sensor is configured to be attached around a rotating shaft exhibiting a magnetostrictive effect and detects torque transmitted by the rotating shaft. The magnetostrictive torque sensor includes a holder being composed of a resin and including a cylinder portion with a hollow cavity in a center through which the rotating shaft is inserted, a flexible substrate that includes a detection portion on which a coil group composed of a combination of a plurality of detection coils aligned in a predetermined direction is formed by a wiring pattern, and a signal line portion on which a plurality of signal lines electrically connecting the coil group to an external device are formed by a wiring pattern, and a cylindrical magnetic ring being composed of a soft magnetic material and being arranged around an outer circumference of the cylinder portion of the holder. The flexible substrate is arranged in such a manner that the detection portion is curved and located between the cylinder portion of the holder and the magnetic ring, and a portion of the signal line portion is led out of the holder. The holder includes an annular wall surrounding an outer circumference of one axial end of the magnetic ring. The magnetic ring includes a retention hole that opens on an outer circumferential surface facing an inner circumferential surface of the annular wall. The holder and the magnetic ring are fixed by a shaft-shaped fastener that penetrates the annular wall and is inserted into the retention hole of the magnetic ring.