Background <p>Little is known about how atypical disease presentations lead to diagnostic errors. Better definitions of atypical presentations may improve our understanding. We aimed to describe how atypical presentations were defined in studies of diagnostic errors in internal medicine.</p> Methods <p>We included papers that described the association between atypical presentations in adult patients and diagnostic errors in internal medicine, indexed from database inception to July 31, 2025. We excluded case reports and conference abstracts. The data were extracted through MEDLINE, Web of Science, CINAHL, Embase, Cochrane Library, Google Scholar, and MedRxiv searches.</p> Results <p>We included 56 papers in this review. Thirty studies included a definition of atypical presentation, but there was a considerable heterogeneity among the definitions. Using basic qualitative content analysis, we developed a new approach (Primary, Suggestive, Uncommon, and Chameleon features—the PSUC approach) to describe clinical presentations and identified four patterns at high risk of diagnostic errors. Pattern 1 lacks Primary disease features (i.e., features always written in textbooks) but has Suggestive features (i.e., stimulating consideration of specific disease); Pattern 2 lacks Primary features but Suggestive and Uncommon features (i.e., uncommon but known features in specific disease) are present. Pattern 3 lacks Primary and Suggestive features but has Uncommon features and Pattern 4 is similar to 3 but with Chameleon features (i.e., primary features for other diseases).</p> Discussion <p>Atypical presentations in studies of diagnostic errors in internal medicine currently have high heterogeneity. A new approach to classify atypical presentations may be useful and warrants investigation in future research.</p> Trial Registration <p>Open Science Framework <a href="http://www.osf.io/27d5m">www.osf.io/27d5m</a>.</p>