Background <p>Family caregivers play a crucial supporting role throughout the disease trajectory of patients with multiple myeloma (MM), and they themselves have supportive care needs (SCN) that are often not being met. The rapid and accurate identification of the met and unmet supportive care needs of family caregivers is an urgent problem.</p> Objectives <p>To develop the Supportive Care Needs and Supply Matching Questionnaire for Family Caregivers of Patients with Multiple Myeloma (SCNS-SM-MM) and to assess its psychometric properties.</p> Methods <p>Thirty-two preliminary items were verified for item generation and content validity. Item analysis was performed using critical ratio, Pearson correlation, corrected item-to-total correlations, commonality, and factor loading. Construct validity was evaluated through Exploratory and confirmatory factor analyses. Reliability was appraised by test-retest reliability, internal consistency, and split-half reliability.</p> Results <p>The questionnaire demonstrated good construct validity, with a four-factor structure (Disease Information, Care Skills, Psychosocial, and Medical Service) identified for the needs assessment. The scale-level content validity index was 0.998. The reliability was proved to be receivable, with a Cronbach’s α of 0.962 and 0.981 for the needs and supply sub-questionnaires, respectively.</p> Conclusions <p>A new questionnaire was developed to assess the matching of supportive needs and supply to family caregivers of patients with multiple myeloma. The results suggest that the questionnaire is a promising tool with good psychometric properties for application in both scientific research and clinical practice.</p> Clinical trial number <p>Not applicable.</p>