<p>Seborrheic dermatitis (SD) is a chronic inflammatory skin condition that is diagnosed clinically, but there is currently no widely accepted, standardized diagnostic criteria. We performed a systematic review and synthesis to summarize and analyze SD diagnostic criteria across published clinical studies to identify common diagnostic approaches and highlight clinical gaps. A systematic literature search was conducted in Embase, PubMed, Scopus, CINAHL Complete, and Cochrane Library in June 2024 in accordance with PRISMA guidelines. Original investigations with a clinical diagnosis of SD were included after deduplication and screening. Data was extracted and analyzed with descriptive statistics, and quality was assessed individually with JBI critical appraisal tools. From 5,548 deduplicated articles, 209 were included for final analysis (&gt; 36,000 SD patients), of which only 22.0% (<i>n</i> = 46) mentioned the use of any clinical criteria to diagnose SD. General skin examination was mentioned in most of these articles. Only 7.7% of studies (<i>n</i> = 16) explicitly described specific diagnostic features for SD, with erythema (16), scale (15), and location (14) most frequently mentioned, and symptoms like pruritus infrequently mentioned (3). Among clinical trials (<i>n</i> = 122), only 5.7% (7) specified diagnostic criteria for inclusion, most with similar reliance on erythema, scale, and location. Most clinical studies of SD, including clinical trials, did not specify any diagnostic criteria. Of the small number of articles that provided specific clinical criteria, only a few physical signs (and not symptoms) were consistently described. These results suggest near universal lack of standardization in the approach to SD diagnosis. Future research should focus on development of consensus, uniformly applied diagnostic criteria in clinical studies.</p>