Background <p>Improving the quality of care for people detained in police custody requires careful measurement and monitoring. However, a standardized and endorsed set of quality indicators for healthcare in police custody settings is lacking. This study aimed to develop a core set of quality indicators to monitor and evaluate police custody healthcare in the Netherlands.</p> Methods <p>A four-phase modified Delphi process was conducted between February 2024 and September 2025: (1) identification of candidate quality indicators ethrough expert brainstorming; (2) rating of indicators on relevance and measurability in a self-administered questionnaire; (3) an in-person group consultation meeting to refine and prioritize the indicators; and (4) content validation of the core set by an advisory committee. A multidisciplinary pool of professionals from healthcare, police, justice, and adjacent sectors participated throughout.</p> Results <p>Brainstorming sessions with 14 experts yielded 185 candidate indicators, which were rated by 19 experts in the questionnaire. The group consultation meeting with 15 participants refined the list, and feedback from 8 advisory committee members informed the final selection. The resulting core set comprised 15 structure, 9–12 process (7 fixed and 2–5 variable), and 17 outcome indicators, covering all key domains of police custody healthcare quality.</p> Conclusions <p>The first evidence-based set of indicators provides a foundation for the systematic assessment and continuous learning in police custody healthcare. Routine healthcare data may serve as a valuable measurement source, though their usefulness depends on data quality, accessibility, and system readiness.</p>