Objectives.We investigated whether sustained attention performance and variability were associated with prefrailty and frailty in the older adult population.Method.A total of 4,317 participants aged 50 years and over from the Irish Longitudinal Study on Ageing (TILDA) completed a comprehensive health assessment. Frailty was defined by low gait speed, low grip strength, unintentional weight loss, self-reported exhaustion, and low physical activity. Scores of greater than or equal to 3, 1-2, and 0 indicated that participants were frail, prefrail, and nonfrail, respectively. Multinomial logistic regression computed associations between frailty state and measures of performance and variability on the Sustained Attention to Response Task (SART). Cognitive processing speed and executive function were also measured. RESULTS: Mean reaction time (RT; odds ratio [OR] = 1.13, p <.05) and RT variability reflective of the top-down aspect of sustained attention (OR = 1.11, p <.05) were associated with prefrailty in the 50-64 age group. Mean RT (OR = 1.72, p <.05) was associated with frailty and RT variability (OR = 1.22, p <.01) with prefrailty in the 65+ age group. Results remained significant following adjustments for cognitive processing speed, executive function, chronic conditions, medications, age, and gender.Discussion.Sustained attention performance and variability were associated with prefrailty and frailty in the older adult population and may represent a novel, objective, and modifiable cognitive marker of frailty progression.