Background: In rehabilitation, therapeutic alliance is associated with improvements in clinical outcomes. The Working Alliance Inventory (WAI) measures therapeutic alliance and is frequently used in rehabilitation research; however, it has not been validated for rehabilitation. Objectives: To determine content validity, internal consistency and construct validity of the Working Alliance Inventory Rehabilitation Dutch Version (WAI-ReD). Methods: In phase 1, content and face validity of the WAI-ReD was judged by professionals (n = 15) and in phase 2 by patients (n = 22). In phase 3, 14 hypotheses were tested in patients (n = 138) regarding: content validity (i.e., missing items, floor, and ceiling effects); internal consistency; and construct validity (i.e., factor structural testing correlations of WAI-ReD scores with Session Rating Scale (SRS), the Helping Alliance Questionnaire II (HAQ-II), and Visual Analog Scale of Pain (VAS(pain))). Results: After phase 1 and phase 2, the WAI-ReD was formulated and tested. Content validity; missing items were negligible. Ceiling effects were present in all domains. Internal consistency; Cronbach's alpha ranged between 0.804 and 0.927. Construct validity; correlations between WAI-ReD, SRS, HAQ-II, and VAS(pain) fell within the hypothesized ranges. Conclusion: Eleven of the 14 hypotheses were not rejected confirming good clinimetric properties of the WAI-ReD. The WAI-ReD can be used in rehabilitation to measure therapeutic alliance.