Background and Objectives: The aging population and the COVID-19 pandemic have led to a rise in severe conditions, including musculoskeletal (MSK) disorders. Although MSK conditions are often managed in primary care, they may sometimes mask serious illnesses requiring urgent diagnosis. The red flag (RF) concept is essential for identifying signs and symptoms of potentially severe disease. However, RF criteria vary across clinical guidelines and lack consistency. With the growing role of direct access to physiotherapy—bypassing physician referral—physiotherapists must develop strong differential diagnostic skills to identify serious pathologies that mimic MSK disorders. This review aims to systematically map how RFs are defined in MSK clinical practice guidelines (CPGs), supporting the move toward a standardized definition for clinical and research use. Materials and Methods: A comprehensive literature search was conducted in PubMed, Web of Science, Scopus, and Cochrane databases. Included studies were CPGs and systematic reviews (SRs) of CPGs addressing MSK disorders and incorporating the RF concept. Data extraction followed a rigorous process, and RF definitions were synthesized and compared in table format. Results: Out of thirteen-thousand three-hundred and ninety-three articles identified, fourteen met inclusion criteria (seven CPGs and seven SRs of CPGs), spanning both physiotherapy and medical fields. All definitions described RFs as signs or symptoms indicating possible serious pathology requiring further investigation or referral. Some definitions referred broadly to “patterns of signs or symptoms”, while others offered more precise criteria. Conclusions: This review highlights the lack of a standardized RF definition in MSK care, leading to inconsistencies in clinical decision-making and diagnosis. To improve patient safety and guide clinicians—especially in direct-access contexts—a unified, internationally recognized definition of RFs is needed in future guidelines.