A magnetic device for gripping and clamping a workpiece in a machining unit or on a machining line comprises a magnetic plate (2) of the type including a distribution of high-energy permanent-magnet modules (3), which can be electrically activated. The device further comprises an auxiliary workpiece-holder plate made of ferromagnetic material, which rests on a main surface of the magnetic plate and has a top surface (10) shaped in a way complementary to a surface of the workpiece (P) that rests thereon. The workpiece (P) rests on the top surface (10) of the auxiliary workpiece-holder plate (8) only in points corresponding to some supporting pads (13), whilst the facing surfaces of the workpiece (P) and auxiliary workpiece-holder plate (8) are kept slightly set apart from one another for most of their extension. Precise referencing in the plane of the main surface (6) of the magnetic plate (2) between the workpiece (P) and the auxiliary workpiece-holder plate (8) is obtained by means of two locating pins (14) projecting from the auxiliary plate (8) and received within respective seats (15) of the workpiece (P). There may be provided a protective shroud (16) above the magnetic plate (2), having an opening (16b) from which the aforesaid auxiliary workpiece-holder plate (8) projects.