Post-traumatic stress disorder (PTSD) is a disabling mental health disorder affecting psychosocial functioning and quality of life. This systematic review is the first to summarize existing global literature on the relationship between PTSD and specific domains of cognitive function in the general population of older adults. We searched PsycINFO, Medline and CINAHL up until November 1st 2024. Studies were included if they were longitudinal cohort or cross-sectional studies of adults aged 60 years and over with a clinical diagnosis of PTSD or subthreshold PTSD symptoms, reporting on any domain of cognitive functioning using a standardised measure. Eighteen good or satisfactory quality articles met criteria for this review, of which three were longitudinal cohort studies and fifteen were cross-sectional studies. PTSD was associated with significant accelerated general cognitive decline and possible accelerated decline in attention and memory over time. Older adults with PTSD additionally performed significantly worse on measures of global cognitive function and memory compared to those without PTSD. For executive function results were mixed; two studies showed significant negative associations, whereas four other studies showed no significant differences between individuals with and without PTSD. Proactive screening of individuals with PTSD for cognitive decline and an additional focus of PTSD treatment on cognitive functioning are needed.