This evolutionary concept analysis explores the meaning of diagnostic overshadowing within the context of physical health care for individuals with mental illness. Diagnostic overshadowing, the misattribution of symptoms of one illness to an already diagnosed comorbidity, leads to compromised patient care and likely contributes to increased mortality experienced by individuals with mental illness. A systematic literature search led to identification of a sample of 25 publications on the topic. Following Rodgers' Evolutionary Concept Analysis methodology, this literature sample yielded unifying definitions, shared themes, factors contributing to the action of diagnostic overshadowing, outcomes caused by this type of misdiagnosis, and possible interventions available to nurses and other healthcare providers. Understanding of the concept diagnostic overshadowing may help prevent its occurrence and its adverse results.