Developing support and an environment for patients with dementia in hospitals is important. This study aims to assess the immediate effect of one-session cognitive stimulation intervention on the mood of patients with dementia in a hospital as preliminary evidence. This study included 33 female patients. The cognitive stimulation intervention was conducted in the day room of the hospital ward by two occupational therapists. The patients participated in one or more sessions. The cognitive stimulation intervention was designed to discuss current affairs that implicitly stimulate memory, executive function, and language skills, according to the cognitive stimulation definition. Outcomes were evaluated using a two-dimensional mood scale. The primary outcome was pleasure. The before and after session scores for the first session and the average score before and after each session at multiple times were compared. The patients' pleasure showed significant improvements in both analyses. These results may indicate that one-session Cognitive stimulation intervention in a hospital effectively improves a mood of pleasure immediately. This study is the first report to provide preliminary evidence on the beneficial alterations of mood after one-session cognitive stimulation intervention for patients with dementia in hospitals. Cognitive stimulation intervention may be an effective non-pharmacotherapy for these patients.