Objective Cancer diagnosis affects patients, their families, and their caregivers in particular. This study focused on the validation of the CareGiver Oncology Quality of Life (CarGOQoL) questionnaire in Portuguese caregivers of patients with multiple myeloma, from the caregiver's point of view. Method This was a cross-sectional study with 146 caregivers of patients with multiple myeloma from outpatient medical oncology and clinical hematology consultations from five hospitals in north and central Portugal. Participants were assessed on quality of life (QoL), psychological morbidity and social support. Result The Portuguese version maintains 17 of the original 29 items version, maintaining general coherence and a dimensional structure that is clinically interpretable. Reliability findings indicated good internal consistency for the total scale (0.86) and respective subscales (0.75 to 0.88), which is in agreement with the alpha values from the previous CarGOQoL validation study for the corresponding subscales (0.74 to 0.89) and total scale (0.90). Significance of results The CarGOQoL is a reliable and valid tool for clinical trials and intervention programs to assess QoL in caregivers of myeloma patients. Future studies should validate the adapted version in caregivers of other types of cancer patients including other chronic diseases.