The aging population worldwide is expected to increase the prevalence of Alzheimer's disease. As there is no medical curative treatment for this disease to date, alternative treatments have been applied to improve the patient's brain and general health. One of these efforts includes providing Alzheimer's patients with proper food and nutrition. In this paper, the authors propose a knowledge-powered personalized virtual coach to provide diet and nutrition assistance to patients of Alzheimer's and/or their informal caregivers. The virtual coach is built on top of an ontology-enhanced knowledge base containing knowledge about patients, Alzheimer's disease, food, and nutrition. Semantics-based searching and reasoning are performed on the knowledge base to get personalized context-aware recommendation and education about healthy eating for Alzheimer's patients. The proposed system has been implemented as a mobile application. Evaluation based on use cases has demonstrated the usefulness of this tool.