A spinous process spacer that is designed to maintain a desired spatial relationship between adjacent vertebrae, is configured for introduction into a spinal implant site in a compressed state and then expands in situ. Once expanded, formations of the present spinal spacer form areas, pockets or spaces that receive at least one bony portion of each adjacent vertebra. The present spinous process spacer has a changeable circumferential profile wherein a first circumferential profile is smaller than a second circumferential profile in order to provide/achieve its compressed and expanded states. The first circumferential profile defines the collapsed position or state while the second circumferential profile defines the position or state. Upon implantation, the present spinous process spacer is not fixed to any bony structure of the vertebrae but provides support. In this regard, use of the spinous process spacer, by itself, will not result in vertebral fusion. However, fusion can result with the use of bone graft packed about the spinous processes (and the spinous process spacer) or in conjunction with the use of an intervertebral body spacer.