Researchers at the University of Exeter Medical School found green spaces helped care home residents relax, and reduced agitation for those with dementia.
The study, published in the Journal of the American Medical Directors Association, considered the findings of 17 different pieces of research.
Lead researcher Rebecca Whear said: "There is an increasing interest in improving dementia symptoms without the use of drugs. We think that gardens could be benefiting dementia sufferers by providing them with sensory stimulation and an environment that triggers memories.
"They not only present an opportunity to relax in a calming setting, but also to remember skills and habits that have brought enjoyment in the past."
Residents at 11 UK care homes were included in the research, as well as services in the US, China and Europe.
Almost half of the elderly people living in residential care have dementia or dementia symptoms.
Despite its positive findings, the study's authors were keen to point out that this area of research is understudied and undervalued by policy makers.
Dr Ruth Garside, an expert in evidence synthesis and one of the paper's authors, said: "There's a lot we don't know about how a garden's design and setting influences its ability to affect well-being, yet it's clear that these spaces need to offer a range of ways of interacting - to suit different people's preferences and needs."