Over 65s are frequent attenders to the Emergency Department (ED) and more than half are admitted for overnight stays. Early assessment and intervention by a dedicated ED-based Health and Social Care Professionals (HSCP) team reduces ED length of stay and the risk of hospital admissions among older adults while improving patient health-related quality-of-life and satisfaction with care. This study aims to evaluate whether augmenting the treatment as usual for older adults admitted to ED is cost-effective. Cost-effectiveness analysis (CEA), conducted alongside the OPTI-MEND randomised controlled trial of 353 patients aged ≥65 with lower urgency complaints compared the effectiveness of early assessment and intervention by a dedicated HSCP team in the ED to treatment as usual (TAU). An economic analysis estimated the average cost per older adults randomised to the HSCP team, and compared to TAU, how contact with HSCP team changed health care use, and associated total costs, and estimated the effect of HSCP on Quality-Adjusted Life Years (QALYs). Within the OPTI-MEND trial, the average cost of a contact with the HSCP team during ED attendance is estimated to be €801 per patient. Compared to TAU, the incremental QALY of intervention is 0.053 (95% CI: 0.023 to 0.0826, p<0.0001). Accounting for cost savings because of contact with HSCP team, the average incremental saving in the total cost, compared to TAU, is -€6,128 (95% CI: -€9,217 to -€3,038, p<0.0001). Given the incremental health gains and significant cost savings, bootstrapped cost CEA suggests that dedicated HSCP care dominates over TAU for low urgency older adults attending the ED. A dedicated HSCP team in the ED significantly improves overall health for lower acuity older adults and, by reducing inpatient length of stay, results in staggering cost savings. This economic evaluation conducted on the OPTI-MEND trial provides convincing evidence that HSCP should be adopted as part of treatment as usual in Irish EDs. ClinicalTrials.gov, NCT03739515; registered on 12th November 2018. https://classic.clinicaltrials.gov/ct2/show/NCT03739515.