OBJECTIVES: Digital contact tracing (DCT) interventions have been deployed at unprecedented scale during COVID-19. However, no comprehensive appraisal of the evidence exists to date regarding their effectiveness. We aimed to systematically review the global literature for a holistic understanding of DCT effectiveness during COVID-19, and to identify factors that enabled or hindered its effectiveness. STUDY DESIGN: Systematic scoping review. METHODS: We searched six databases for peer-reviewed literature relevant to the evaluation of DCT interventions during COVID-19 (January 2024) (CRD42021268586). We compiled implemented DCT interventions from grey literature. Effectiveness appraisals, different operationalizations, measurements, and definitions of DCT effectiveness, as well as associated factors were synthesized qualitatively. Study quality was assessed using the Mixed Methods Appraisal Tool. We followed Cochrane and PRISMA guidance. RESULTS: We identified 133 studies evaluating 121 different DCT implementations. Seventy-three (60 %) studies found DCT to be effective, mostly when evaluating epidemiological impact metrics. Public trust emerged as crucial for DCT to be effective, which requires high and enforceable data safety and privacy standards, clear and transparent communication, high accuracy and reliability of the intervention, and an acceptance-enhancing implementation approach of other pandemic response measures by public health authorities more broadly. Most evaluations took place in high rather than low-resource settings. CONCLUSION: While technical performance matters, DCT effectiveness primarily depends on a relatively small number of non-technical drivers centred around public trust. DCT should only be implemented as integrated part of a broader public health framework. Our findings hold important insights for the design, implementation, and evaluation of other digital technology for pandemic response.