This study examined how patients with cancer estimate caregiver burden (CB) and the association between their underestimation of CB and their caregivers' self‐ratings of their quality of life (CQOLC‐K; Korean version of the Caregiver Quality of Life Index‐Cancer), depression and anxiety (Korean version of the Hospital Anxiety and Depression Scale). Participants consisted of 990 patient‐caregiver dyads recruited from a nationwide cross‐sectional survey conducted in South Korea. Medical baseline data were retrieved from the hospital information systems of the participating centres. The patients with cancer who underestimated CB ranged from 18.62% (for physical CB) to 23.33% (for social CB). They had less advanced cancer, a lower income, were the caregiver's spouse, reported higher levels of family avoidance of communication about cancer, and had female caregivers. The patients' underestimation of CB was significantly related to lower CQoL and higher levels of caregiver depression and anxiety. The current study provides empirical evidence for the link between the underestimation of CB by patients with cancer and compromised caregiving experiences of cancer caregivers. Open family communication about cancer was discussed as one of several practical strategies for decreasing patients' underestimation of CB.