We assessed the association between blood methylmercury (MeHg) and cognitive function in 240 adult residents living near a deserted chloralkali plant. Total mercury (T-Hg) in the blood, MeHg, and health and dietary related questionnaire were examined for all participants. The Cognitive Abilities Screening Instrument (CASI C-2.0) and Mini-Mental State Examination (MMSE) were used to assess the participants’ cognitive functions. We found a significantly high correlation (r=0.979; p<0.001) between blood T-Hg (17.3±10.9 μg/L) and MeHg (15.3±9.2 μg/L). We also found significantly higher blood MeHg levels in participants with high local fish and seafood consumption, which revealed that dietary intake was the major exposure route of MeHg. All the participants were assigned to the high-MeHg (H-MeHg, 27.0±10.4 μg/L) or low-MeHg (L-MeHg, 11.6±4.7 μg/L) groups based on the 75th percentile of their blood MeHg (19.2 μg/L), and then matched for cognitive function confounders: age, gender, and education levels. Higher abnormality rates for remote memory (p=0.036), mental manipulation (p=0.013), and orientation (p=0.005) were found in the H-MeHg group than in the L-MeHg group. Long-term consumption of MeHg-contaminated fish and seafood by residents living near this contaminated area may have persistent effects on their cognitive function. We suggest a follow-up study to monitor the long-term health effects on the residents living near this deserted plant.