Background: Dementia of Alzheimer’s disease (AD) imposes burdens on patients, caregivers, and society. This cross-sectional study examined caregiver-reported history of disease onset and AD dementia to inform efforts promoting early disease detection and diagnosis. Methods: An online survey collected self-reported cross-sectional data – demographic characteristics, diagnosis, treatment experiences, and other information on disease detection, diagnosis, and treatment – from caregivers of patients with AD dementia. These characteristics were examined as a function of AD dementia severity. Results: Three hundred patients with AD dementia were trichotomized by long-term care insurance levels reported by caregivers: 12.3% (n=37) as low severity, 63.7% (n=191) as medium severity, and 24.0% (n=72) as high severity. The Short-Memory Questionnaire and patient dependency scores both varied significantly across severity groups. AD dementia symptoms were most frequently first detected by a caregiver (58.7%) or the patient’s family (45.7%). However, in 13.7% of cases, symptoms were detected by a health care provider during a routine visit. Memory problems were the most frequent first symptoms (77.3%), followed by repetition (55.7%). Patients (73.7%) were taking symptomatic treatment such as acetylcholinesterase inhibitors or memantine. High-severity patients were older, had been diagnosed with AD dementia for a longer time, had more frequent reports of memory problems as the first symptoms detected, and required more hours of care per day, compared with low-severity patients. Conclusion: Caregivers and families play an integral role in the identification of AD dementia patients, with memory problems being common first symptoms noticed by caregivers that led to a diagnosis of AD dementia. These results provide novel insight into the detection, diagnosis, and treatment of AD dementia in Japan and how these factors differ across the spectrum of disease severity.