Increased carotid intima-media thickness (C-IMT), a marker of atherosclerosis, is known to be associated with cerebrovascular and cortical abnormalities and cognitive impairment. This prospective observational study investigated the association between increased C-IMT and postoperative cognitive dysfunction (POCD) in older adult patients undergoing off-pump coronary artery bypass surgery (OPCAB). A total of 201 patients (57 females, 144 males; >60 years) were classified into increased (n = 105) or normal (n = 96) C-IMT groups by a cut-off value of 0.9 mm (bilateral C-IMT mean). Cognitive function was serially assessed with the Korean Mini-Mental State Examination (K-MMSE), and Korean Montreal Cognitive Assessment (K-MoCA) scores preoperatively and at 7 d and 3 months postoperatively. POCD was defined as the deterioration of 1 standard deviation in at least one postoperative tests compared with their corresponding baseline scores. Independent risk factors for POCD were evaluated using multivariable analysis. Overall, POCD occurred in 46 patients (23%) over the 3 months. The incidences of POCD at 7 d and 3 months after surgery were similar, and there was no difference in both K-MMSE and K-MoCA test scores before and after surgery between groups. Chronic obstructive lung disease and intraoperative hyperglycemia episodes (>180 mg/dl), but not increased C-IMT, were independent risk factors for POCD. Unlike in non-surgical cohorts, increased C-IMT was not significantly associated with the occurrence of POCD in older adult patients undergoing OPCAB.