Individuals with anxiety disorders exhibit lower intrinsic functional connectivity between prefrontal cortical areas and subcortical regions. The prefrontal cortex (PFC) is sensitive to the acute and chronic effects of physical activity (PA), while the anxiolytic effects of PA are well known. The current study examined the association of generalised anxiety disorder (GAD), and its interaction with PA, with resting-state left PFC oxygenation. This cross-sectional study used data from participants (n=2444) from The Irish Longitudinal Study on Ageing, a nationally representative prospective study of community-living adults aged ≥50 years in Ireland. The Composite International Diagnostic Interview Short-Form determined fulfilment of criteria for GAD. The short-form International Physical Activity Questionnaire determined adherence to the World Health Organisation PA guidelines. Resting-state, left PFC oxygenation was continuously measured via a Portalite. Tissue saturation index (TSI) was calculated as the ratio of oxygenated haemoglobin to total tissue haemoglobin (expressed as a percentage) for the final minute of a five-minute supine-rest period. Multivariable lineae regression quantified associations of GAD with TSI in the total population and population stratified by PA status. Participants with GAD had lower TSI (b=-1.416, p=0.008) compared to those without GAD. However, this association was modified by PA. Among participants who met the PA guidelines, TSI did not differ according to GAD status (b=-0.800, p=0.398). For participants who did not meet the guidelines TSI was significantly lower among those with GAD (b=-1.758, p=0.004). These findings suggest that PA may help to protect brain health among older adults with GAD.