The purpose of this study was to examine the relationship between knowledge and attitudes towards dementia among adults. A cross-sectional online survey with purposive sampling was conducted in four district health centers in Taichung, Taiwan, in 2018. Knowledge was measured by the Taiwanese version of the Dementia Knowledge Assessment Scale. Attitudes towards people with dementia were measured by four kinds of feelings: discomfort, shame, avoidance, and fear. In total, 347 persons completed the questionnaire. Knowledge of dementia was modest. Higher education, a care-related background, chronic health conditions, experience caring for people with dementia, and knowledge of family history were related to higher knowledge of dementia. Attitudes towards people with dementia were grouped into four clusters: uncomfortable (22.2%), ashamed (2.6%), unfriendly (22.5%), and non-negative (52.7%). Knowledge of dementia was significantly related to the ashamed cluster but not to the other clusters. Lower education, a lack of caring experience, and a lack of knowledge of family history were related to uncomfortable feelings, and poorer financial satisfaction was related to uncomfortable, afraid, and avoidant feelings. The open-question expression of feelings towards dementia was more likely to be negative (55.3%). The public should be educated on knowledge of and friendly attitudes towards dementia.