Background: Although the general-care knowledge of family caregivers related to dementia had been studied, little attention has been given to understanding the knowledge of caregivers related to dementia diet nutrition. This knowledge is important to help caregivers handle the eating and nutrition problems of people with dementia (PwD). Purpose: The purpose of this study was to explore the dementia diet nutrition knowledge of family primary caregivers and to analyze whether demographic characteristics of caregivers and/or the dementia stages of PwD have a relationship with dementia diet nutrition knowledge. Methods: A descriptive and cross-sectional study using a questionnaire survey was conducted. A total of 220 primary family caregivers of people with dementia were recruited. Results: Results showed that the average total score for the 13 questions on diet nutrition knowledge was 71.85%. Moreover, only 28.2% of the participants answered the question “Malnutrition is a significant risk during the early stage of dementia” correctly. In addition, the findings showed significant differences in the diet nutrition knowledge of participants based on education level (F = 3.989, p < .05) and the use of foreign (non-Taiwanese) caregivers (t = -2.919, p < .01). Conclusions: The participants had sufficient overall diet nutrition knowledge of dementia. However, they lacked knowledge in specific areas. Family caregivers with higher levels of education and those who hired foreign caregivers had better knowledge scores. The results may be used as a reference for nurses providing dementia dietary education. It is recommended that special attention be given to dietary education when patients are at the early onset phase of the disease and when caregivers have a lower level of education in order to improve the overall quality of dementia dietary care in the community.