Background: There is a paucity of evidence on the efficacy of garden design based on dementia-friendly environment (DFE) characteristics on the level of agitation, apathy, and engagement of people with dementia in residential aged care facilities (RACFs). Objective: To investigate the effect of a garden improved according to DFE characteristics on agitation, apathy, and engagement of people with dementia in one RACF. Methods: A case study design with a mixed-method approach was used. Results: There was no significant improvement in the level of participants’ agitation following visits to the improved garden, χ2(2) = 5.167, p =.076. A high level of engagement was found in participants during the intervention (Week 1-1, p <.01; Week 1-2, p <.01; Week 2, p <.01; Week 3, p <.05; and Week 4, p <.05) when compared to before intervention (Week 0). A higher level of apathy was found in participants at Week 0 when compared to during the intervention (Week 1-1, p <.05; Week 1-2, p <.01; Week 2, p <.05; Week 3, p <.01; and Week 4, p <.01). Five themes emerged from participant interviews: the presence of sensory-provoking elements in the garden, meaningful engagement in the garden, accessibility of the garden, garden impacts, and garden experiences that demonstrated the effectiveness of the garden. Conclusions: The garden promoted engagement and decreased apathy of people with dementia living in the RACF with the researcher’s partial facilitation of the intervention sessions. The qualitative findings indicated the effectiveness of the garden in reducing agitation.