An interspinous spacer assembly for insertion and/or implantation between spinous processes of adjacent superior and inferior vertebrae includes an interspinous spacer member sized and configured for insertion into the interspinous space located between adjacent spinous processes and an engagement mechanism for operatively coupling the spacer member to the adjacent spinous processes and for preventing migration of the assembly once implanted. The interspinous spacer assembly is adjustable to conform to the individual anatomy of a patient's spine.