Background: The use of digital technology might contribute to social engagement and mental health in an ageing population. Objective: To systematically map and describe the scientific knowledge and knowledge gaps on the effects of different types of digital tools for outcomes related to social engagement in older adults at risk for mental health decline. Methods: Thirteen groups of outcomes (domains) were defined, related to social engagement, by the use of digital tools. Thereafter, a systematic literature search was conducted in April 2017 to identify systematic reviews in line with the objective, an additional search for systematic reviews published since April 2017 was done in September 2019. In addition, reference lists as well as protocols from the Cochrane Collaboration and Campbell were searched. The final sample includes nine systematic reviews, published between 2012 and 2019, with both qualitative and quantitative results. Results: This evidence map shows that education in and use of digital tools, including computers and the internet, might decrease loneliness. Research is still limited, but the existing research points towards positive effects from the use of digital tools on outcomes related to social engagement, such as loneliness, social isolation, social interaction and/or social support, participation, depression, physical health/activities, as well as security, self-esteem or empowerment. Scientific knowledge gaps are present for the domains of quality of life, activities in daily living (ADL), cognition, autonomy, and usability. Conclusion: The evidence map shows inconclusive evidence about the effects on outcomes related to social engagement from the use of digital tools for older adults at risk of mental health decline or mental illness. More research is needed, and future research should include studies with a strong design, larger populations, and various subgroups of older adults in different contexts to further evaluate the effects of the digital tools related to social engagement for older adults at risk of mental health decline or mental illness.