Background: Addenbrooke's Cognitive Examination III (ACE-III) was developed as a screening tool for cognitive disorders. Many countries have proven the cultural adaptation, reliability and validity of ACE-III. Aims: To make cultural adaptations of ACE-III for the Turkish population and to examine its validity and reliability. Methods & Procedures: First, ACE-III was translated and adapted into Turkish (ACE-III-TR), then its validity and reliability were examined. The study included 234 people: 93 with dementia (78 Alzheimer's disease (AD) and 15 frontotemporal dementia (FTD)), 46 with mild cognitive impairment (MCI) and 95 healthy. Two blinded speech and language therapists rated the ACE-III-TR simultaneously for interrater validity. The same practitioner retested the same participants 2 weeks later for test-retest reliability. The construct validity of the culturally adapted test was assessed by analysing subsection correlations with the ACE-III-TR total score. The association between the Mini-Mental State Examination (MMSE) total score, relevant subsections and ACE-III-TR total score was examined for criterion validity. Intergroup differences for healthy, MCI and dementia were studied for ACE-III-TR subsections and total score, and cut-off scores were calculated for total score with sensitivity and specificity in differential diagnosis. Results & Outcomes: Attention, memory and ACE-III-TR total scores showed a statistically significant difference between the three groups of dementia, MCI and healthy (p < 0.001). Statistically significant positive correlations ranging from 0.571 to 0.929 were found between ACE-III-TR subsections and total scores (p < 0.05). A highly significant positive correlation was found between MMSE total score and ACE-III-TR total score (r = 0.870). Between the second and first measurements, positive, moderately significant correlations were found for all subsections and ACE-III-TR total (ICC = 0.508-0.784, r = 0.477-0.646). A high level of agreement was found between two raters for all ACE-III-TR subsections and the ACE-III-TR total score (alpha = 0.9296-0.99995). The total ACE-III-TR cut-off score was 79.5 for healthy and MCI and 69.5 for MCI and mild stage dementia. Conclusions & Implications: This study found that ACE-III-TR is a sensitive and specific screening test for the diagnosis of MCI and dementia that has high validity and reliability. ACE-III-TR was found to be a valid and reliable tool in dementia, including AD and FTD, and in mild, moderate and advanced dementia. By providing a more comprehensive assessment of a person's cognitive profile, it can help the clinician make a differential diagnosis of MCI and dementia. ACE-III-TR may be useful in monitoring the progression of cognitive deficits in clinical practice, research studies and therapy follow-up processes.