Background: Mucopolysaccharidosis type III (MPS III) is a rare lysosomal storage disease with systemic complications. This scoping review aimed to synthesise evidence regarding methods to diagnose and monitor MPS III. Methods: We searched 10 databases for English and Spanish citations published from 2017 to 2022. Our study focused on human-based research that reported methods to diagnose and monitor MPS III, including experimental, quasi-experimental, observational studies, reviews, and guidelines. We followed PRISMA-ScR guidelines for screening and data extraction. Descriptive statistics and qualitative synthesis were used for analysis. Results: We included 35 eligible articles. Most studies were conducted in Europe and Central Asia (17/35, 48.6%) and constituted case reports (19/37, 51.4%). Clinical symptoms were reported in 30 articles and predominantly pertained to the nervous system (25/30, 83.3%), including intellectual disability (19/30, 63.3%), movement incoordination (17/30, 56.6%), and behavioural issues (16/30, 53.3%). Diagnostic methods (23/35, 65.7%) featured genetic testing and biochemical assays, including GAG measurement in urine. Brain MRI was the most recorded imaging study (11/20, 55.0%), while electrographic studies (10/35, 28.5%) encompassed electrocardiogram (5/10, 50%), electroencephalogram, and audiometry (3/10, 30% each). Scales and questionnaires (8/35, 22.8%) were reported, with the Bayley Scales of Infant Development being the most described (4/8, 50%). Conclusion: Our study comprehensively overviews the contributions of diagnostic and monitoring methods for MPS III. Our findings can guide clinicians in providing evidence-based care for this rare disease.