Among psychosocial factors affecting emotional adjustment and quality of life, social support is one of the most important and widely studied in cancer patients, but little is known about the perception of support in specific significant relationships in patients with cancer. This study examined the psychometric properties of the Quality of Relationship Inventory (QRI) by evaluating its factor structure and its convergent and discriminant validity in a sample of cancer patients. A total of 388 patients completed the QRI. Convergent validity was evaluated by testing the correlations between the QRI subscales and measures of general social support, anxiety and depression symptoms. Discriminant validity was examined by testing group comparison. The QRI's longitudinal invariance across time was also tested. Principal axis factor analysis with promax rotation identified three factors accounting for 42.99% of variance: perceived social support, depth, and interpersonal conflict. Estimates of reliability with McDonald's ω coefficient were satisfactory for all the QRI subscales (ω ranging from 0.75 – 0.85). Satisfaction from general social support was negatively correlated with the interpersonal conflict subscale and positively with the depth subscale. The interpersonal conflict and social support scales were correlated with depression and anxiety scores. We also found a relative stability of QRI subscales (measured 3 months after the first evaluation) and differences between partner status and gender groups. The Quality of Relationship Inventory is a valid tool for assessing the quality of social support in a particular relationship with cancer patients.