Objectives: This study had two purposes: to develop an instrument for assessing family members' caregiving needs before their older relative's knee joint-replacement surgery and to determine instrument psychometrics. Methods: In phase 1 of this validation study, we generated 34 items based on previous interviews with 138 family caregivers of patients with knee osteoarthritis (OA), an intensive literature review, and the authors' clinical experiences. In phase 2, we examined the content and face validities of the 34-item Family Members' Caring Needs Inventory (inventory) to develop a 32-item inventory. In phase 3, 150 family members of outpatients with knee OA were recruited from three hospitals in northern Taiwan and surveyed with the inventory to determine its internal consistency reliability and test–retest reliability. Results: The final 30-item inventory had excellent content and face validities. Its factor analysis yielded a five-factor solution, accounting for 82.9% of the variance. The inventory had Cronbach's α = 0.97 and intraclass correlation coefficient = 0.93, indicating very high internal consistency reliability and test–retest reliability. The inventory was perceived as easy to complete and yielded highly acceptable validity and reliability levels. After cross-cultural adaptation, this tool may be used to assess family members' caregiving needs before their relative's knee-replacement surgery. The role of family members and spouses in supporting patients with osteoarthritis (OA) is crucial. After cross-cultural adaptation, the Family Members' Caring Needs Inventory may be used by health care providers to assess and provide relevant information to meet the needs of family members caring for an older relative with knee OA. Conclusions: This assessment and specific caregiving information for family members of older knee OA patients may promote patients' quality of life and decrease their OA-related burden.