Primary family caregivers of patients with dementia often experience high caregiver burden and significant decline in a range of health outcomes. The current study examined the relationship between medical comorbidities of inpatients with dementia and caregiver burden in adult–child primary caregivers, and the buffering effect of having a secondary caregiver on the relationship between patients' comorbidities and caregiver burden. The study is a secondary analysis of data from a cross‐sectional observational study design. The sample comprised 477 dyads of inpatients with dementia and adult–child primary caregivers attending the neurological department of two grade A hospitals. All the inpatients were assessed with the Charlson Comorbidity Index (CCI) and the Mini‐Mental State Examination (MMSE). All the adult–child primary caregivers were assessed with the Zarit Burden Interview (ZBI) and completed the questionnaires on socio‐demographic data, caring hours, presence of secondary caregiver, and the level of impairment of the patient. Higher burden was associated with higher scores on the CCI and having a spouse of the patient as the secondary caregiver. A significant interaction occurred between the CCI and caregiver burden when having a spouse as the secondary caregiver, indicating the negative effect of the CCI on caregiver burden was greater when the spouse of the patient served as the secondary caregiver. In summary, the negative impact of patients' comorbidities on caregiver burden in adult–child primary caregivers was heightened when the secondary caregiver role was undertaken by the spouse of the patient with dementia. These results may inform programmes targeted to improve care arrangements for people with dementia and their caregivers.