A piston for a magneto-rheological (MR) damper includes an upper core 11 having a side opening 113; a lower core 12 having a lower core body 121 and a central protrusion 122 protruding upwardly from the lower core body 121; and a magnetic field generator 13 coupled to the central protrusion 122, including a body 131A around which a coil 132 is wound, a ring-shaped upper support 131B extending from the upper portion of the body 131A in the radial direction, and a ring-shaped lower support 131C extending from the lower portion of the body 131A in the radial direction.