Abstract Background Malnutrition is common among older adults and is associated with adverse outcomes but remains undiagnosed on healthcare admissions. Older adults use emergency departments (EDs) more than any other age group. This study aimed to determine the prevalence and factors associated with malnutrition on admission and with adverse outcomes post-admission among older adults attending an Irish ED. Methods Secondary analysis of data collected from a randomised controlled trial exploring the impact of a dedicated team of health and social care professionals on the care of older adults in the ED. Nutritional status was determined using the Mini Nutritional Assessment- short form. Patient parameters and outcomes included health related quality of life, functional ability, risk of adverse health outcomes, frailty, hospital admissions, falls history and clinical outcomes at index visit, 30-day and 6-month follow up. Aggregate anonymised participant data linked from index visit to 30-days and 6-month follow-up were used for statistical analysis. Results Among 353 older adults (mean age 79.6 years (SD = 7.0); 59.2% (n = 209) female) the prevalence of malnutrition was 7.6% (n = 27) and ‘risk of malnutrition’ was 28% (n = 99). At baseline, those who were malnourished had poorer quality of life scores, functional ability, were more frail, more likely to have been hospitalised or had a fall recently, had longer waiting times and were more likely to be discharged home from the ED than those who had normal nutrition status. At 30-days, those who were malnourished were more likely to have reported another hospital admission, a nursing home admission, reduced quality of life and functional decline than older adults who had normal nutrition status at the baseline ED visit. Differences between the MNA SF and 6-month outcomes were similar but not statistically significant. Conclusion Over one-third of older adults admitted to an Irish ED are either malnourished or at risk of malnourishment. Malnutrition was associated with a longer stay in the ED, functional decline, poorer quality of life, increased risk of hospital admissions and a greater likelihood of admission to a nursing home at 30 days. Trial registration Protocol registered in ClinicalTrials.gov, ID: NCT03739515, first posted November 13, 2018.