This systematic review investigated evidence for the effectiveness of trained assistance dogs as an intervention and support for people with dementia. Peer‐reviewed articles were retrieved from multiple databases (SCOPUS, Web of Science, and Google Scholar). Reference lists of the articles retrieved were also screened to identify key authors for inclusion. Articles were subject to a quality review, and the results synthesised to address the research questions. Both qualitative and quantitative data were reported. There was evidence of the potential positive effects of dog‐assisted interventions in relation to mood, prosocial behaviours, activities of daily living and/or quality of life, cognitive impairment and symptoms specific to dementia, and existential functions. However, the form and quality of the studies varied considerably. It was concluded that further research is needed, particularly for those with younger onset dementia (YOD), for whom there were no specific studies available. Attention needs to be given to establishing adequate‐sized samples and designs incorporating control groups. Research would benefit from employing mixed‐methods, allowing for the triangulation of qualitative and quantitative data. For deteriorating conditions such as the dementias, studies need to be conducted over time. Furthermore, given the important place family members play in the lives of those with dementia, the effects of dog‐assisted interventions on the wider family should also be considered.