Dementia causes a great burden of disease globally. Volunteers’ contributions in caring for older persons with dementia (OPD) are growing. This review aims to evaluate the impact of trained volunteers’ involvement in providing care and support for OPD. PubMed, ProQuest, EBSCOHost, and Cochrane Library databases were searched using specific keywords. Inclusion criteria were studies of OPD who received interventions delivered by trained volunteers, which were published between 2018 and 2023. Seven studies were included in the final systematic review, comprising studies using quantitative and qualitative approaches. A wide range of outcomes was seen in both acute and home/community-based care settings. Improvements in social interaction, loneliness, mood, ability to recall, and physical activity of OPD were found. Carers and trained volunteers were also found to benefit. Trained volunteers’ involvement in OPD care plays a very valuable role for OPD, their caregivers, the volunteers, and in turn to society. This review also emphasizes the importance of person-centred care for OPD.