A positioning apparatus configured to actively assist the support, positioning, or support and positioning of a patient on a patient support apparatus is provided. The positioning apparatus includes a sliding bearing unit to allow the positioning apparatus to be supported in a movable manner on the patient support apparatus, and a bearing unit with at least two bearing regions. At least a first of the at least two bearing regions is configured to support and/or provide support on the patient support apparatus, and at least a second of the at least two bearing regions is configured to assist the positioning of the patient.