Purpose: This study investigated the feasibility of a one-on-one peer support intervention in family caregivers of newly diagnosed patients with a hematological cancer during initial treatment. Methods: The study was a one-arm feasibility study including family caregivers of newly diagnosed patients with hematological cancer (n = 26) and caregiver ambassadors who were family caregivers of previously treated patients as peer supporters (n = 17). The one-on-one peer support intervention consisted of three components: a caregiver ambassador preparatory course; 12 weeks of one-on-one peer support, and caregiver ambassador network meetings. Results: Family caregivers reported high satisfaction with the delivery and flexibility of one-on-one peer support and improved in most psychosocial outcomes over time. Telephone and text messages were the most used form of contact between the peers. Caregiver ambassadors reported high satisfaction with the preparatory course and used the available support from the network meetings. No adverse events were reported. Conclusion: One-on-one peer support provided by a caregiver ambassador is feasible and safe in family caregivers of newly diagnosed hematological cancer patients during their initial treatment. Utilizing volunteer caregiver ambassadors has the potential to be a new support model in family caregivers of hematological cancer patients across diagnostic groups within a clinical setting. Clinical trial registration number: NCT04039100, July 29, 2019.