Among elderly without cognitive impairment, poor physical performances have been reported to predict cognitive decline and dementia. Our aim was to explore the predictive value of balance impairment for cognitive decline in 686 community-dwelling Alzheimer's disease (AD) patients (REAL.FR study). Being unable to stand on one leg for five seconds or more defined balance impairment. Cognitive decline was assessed using the Mini-Mental Status Examination (MMSE) score. Co-morbidities, behavioral and psychological symptoms of dementia (BPSD) using the Neuropsychiatric Inventory score, medication, and level of education were assessed at the hospital. MMSE and balance were reported every six months during two years. Linear mixed model analyses were performed. At baseline, participants with balance impairment (15.2% of the sample) were significantly older, had a lower MMSE score and more BPSD, co-morbidities, and medication. After adjustment for the potential covariates, the presence of balance impairment at each assessment was associated with a mean MMSE decline of 9.2 (1.4) points at two years; having no balance impairment at each assessment was associated with a mean MMSE decline of 3.8 (0.3) points at two years (p < 0.001). An abnormal one-leg balance test is a marker of more advanced dementia and predicts a higher rate of cognitive decline.