BACKGROUND: Persons seeking emergency injury care are often from higher-risk and underserved key populations (KPs) and priority populations (PPs) for HIV programming. While facility-based HIV Testing Services (HTS) in Kenya are effective, emergency department (ED) delivery is limited, despite the potential to reach underserved persons. METHODS: This quasi-experimental prospective study evaluated implementation of the HIV Enhanced Access Testing in Emergency Departments (HEATED) at Kenyatta National Hospital ED in Nairobi, Kenya. The HEATED program was designed as a multi-component intervention employing setting appropriate strategies for HIV care sensitization and integration, task shifting, resource reorganization, linkage advocacy, skills development and education to promote ED-HTS with a focus on higher-risk persons. KPs included sex workers, gay men, men who have sex with men, transgender persons and persons who inject drugs. PPs included young persons (18-24 years), victims of interpersonal violence, persons with hazardous alcohol use and persons never HIV tested. Data were obtained from systems-level records, enrolled injured patient participants and healthcare providers. Systems and patient-level data were collected during a pre-implementation period (6 March - 16 April 2023) and post-implementation (period 1, 1 May - 26 June 2023). Additional, systems-level data were collected during a second post-implementation (period 2, 27 June - 20 August 2023). HTS data were evaluated as facility-based HIV testing (completed in the ED) and distribution of HIV self-tests independently, and aggregated as ED-HTS. Evaluation analyses were completed across reach, effectiveness, adoption, implementation and maintenance framework domains. RESULTS: All 151 clinical staff were reached through trainings and sensitizations on the HEATED program. Systems-level ED-HTS among all presenting patients increased from 16.7% pre-implementation to 23.0% post-implementation periods 1 and 2 (RR = 1.31, 95% CI: 1.21-1.43;  < 0.001). Among 605 enrolled patient participants, facilities-based HTS increased from 5.7% pre-implementation to 62.3% post-implementation period 1 (RR = 11.2, 95%CI: 6.9-18.1;  < 0.001). There were 440 (72.7%) patient participants identified as KPs (5.6%) and/or PPs (65.3%). For enrolled KPs/PPs, facilities-based HTS increased from 4.6% pre-implementation to 72.3% post-implementation period 1 (RR = 13.8, 95%CI: 5.5-28.7,  < 0.001). Systems and participant level data demonstrated successful adoption and implementation of the HEATED program. Through 16 wk post-implementation a significant increase in ED-HTS delivery was maintained as compared to pre-implementation. CONCLUSIONS: The HEATED program increased overall ED-HTS and augmented delivery to KPs/PPs, suggesting that broader implementation could improve HIV services for underserved persons already in contact with health systems.