Delirium is a serious and prevalent problem that occurs in many hospitalized older adults. Delirium superimposed on dementia (DSD) occurs when a delirium occurs concurrently with a pre-existing dementia. DSD is typically underrecognized by medical and nursing staff. The current study measured nursing identification of DSD using standardized case vignettes, and the Mary Starke Harper Aging Knowledge Exam (MSHAKE). Results revealed that the nurses in this study had a high level of general geropsychiatric nursing knowledge as measured by the MSHAKE, yet had difficulty recognizing DSD compared to dementia alone and delirium alone. Only 21% were able to correctly identify the hypoactive form of DSD, and 41% correctly identified hypoactive delirium alone in the case vignettes. Interventions and educational programs designed to increase nursing awareness of DSD symptoms could help to decrease this gap in nursing knowledge.