Alzheimer's disease (AD) is a progressive neurodegenerative disease leading to a gradual and irreversible loss of memory, linguistic skills, and perception of time and space, thinking, and behavior. Dietary pattern has been presented as a contributor to the incidence of Alzheimer's. This study aimed at reviewing the evidence on the relation between dietary pattern and AD. This systematic search was performed on the articles available in PubMed, Scopus, and Web of Sciences databases until May 2019 using keywords, including (diet, food, dietary pattern, food pattern) and (Alzheimer's disease) among observational studies. After excluding duplicated, and irrelevant studies, 26 studies were eligible for this review study. We categorized the studied dietary patterns into two groups: healthy and unhealthy diet. This study reviewed two case-control, five cross-sectional, and 19 prospective studies. Eight studies assessed unhealthy diet (high-fat diet, high-glycemic diet, sweetened sugary beverage, etc.) and the risk of AD. In addition, the other studies considered the effect of healthy diet such as Mediterranean diet, dietary approaches to stop hypertension (DASH), Mediterranean-DASH intervention for neurodegenerative delay, and seafood-rich diet on AD. This literature review indicated that adherence to a healthy dietary pattern has neuroprotective effects on AD prevention, while unhealthy diet can cause neurodegenerative effects in AD etiology. In conclusion, our findings showed that adherence to healthy diet can decrease oxidative stress and inflammation and accumulation of amyloid-β and consequently can decrease the risk of AD.