Objective: To synthesize the evidence on the associations between vegetarian and/or vegan diets (VVDs) and symptoms of orthorexia nervosa (ON) compared with omnivorous diets in the adult population. Method: Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and the Meta-analyses of Observational Studies in Epidemiology (MOOSE) guidelines, we conducted a systematic review and meta-analysis. We searched the MEDLINE/PubMed, Embase/Scopus, PsycINFO, and Web of Science databases up to June 17, 2025, with no language or date restrictions. Random effects models with the Sidik–Jonkman method were used to estimate pooled effect sizes. Results: The meta-analysis included 26 cross-sectional studies with a total of 23,783 participants (72.0% female; mean age range: 19.6–51.0 years). Adults who followed VVDs had moderately higher ON symptoms compared to omnivores (standardized mean differences using Cohen's d index = 0.46; 95% confidence interval [CI]: 0.33, 0.60; inconsistency index [I2] = 81.0%). Additionally, categorical data revealed that VVD adherents were approximately twice as likely to report ON symptoms as omnivores (odds ratio = 1.99; 95% CI: 1.21–3.25; I2 = 92.8%). Vegetarians and vegans were similarly associated with ON symptoms compared with omnivorous (p = 0.855).