The axially adjustable magnetic bearing comprises an annular body secured to a support, a magnetic bearing stator structure having an annular sheath for supporting electromagnet windings placed concentrically relative to said body, and an annular rotor armature placed concentrically relative to the stator structure, leaving an airgap relative thereto. The annular body includes at least one radial slot associated with clamping means, and includes a set-back portion in its face facing towards the stator structure. The face of the annular body facing towards the stator structure includes a sliding surface that co-operates with a facing portion of said sheath, and the set-back portion of the annular body presents a threaded portion that co-operates with a threaded portion of an adjustment ring engaged in an annular groove formed in the face of the sheath that faces towards the body. The adjustment ring is prevented from moving in the axial direction in said annular groove.