Despite the widespread adoption and expansion of DNA databases by countries around the world, few studies have investigated the factors that promote public support or opposition to their expansion. Drawing on the normative and instrumental models of police legitimacy, this study explored the mechanisms contributing to public support for expanding DNA databases in a national sample of approximately 1,000 adults in South Korea. Using structural equation modelling, we examined the direct and indirect effects of citizen perceptions of procedural justice, police effectiveness, and police legitimacy on citizen support for expanding DNA databases, a form of police empowerment. Our results indicated that trust in the police was a key antecedent of citizen support for the expansion of DNA databases. Both citizen perceptions of procedural justice and police effectiveness indirectly increased public support for expanding DNA databases via increased trust in the police, but neither had a direct impact. Notably, procedural justice had a stronger indirect impact than police effectiveness on public support for the expanded use of DNA technology. Our study emphasises the importance of law enforcement agencies’ efforts to build trust and legitimacy in the eyes of the public to enhance support for expanding DNA databases. This can be achieved by treating citizens in a procedurally fair manner and effectively addressing crime, with a greater emphasis on the former.