<p>This systematic review aimed to evaluate the association between diabetes mellitus (types 1 and 2) and choroidal thickness (CT) as measured by optical coherence tomography (OCT). A systematic literature search was conducted across PubMed, Embase, Web of Science, Scopus, and the Cochrane Library for observational studies published from 2010 to 2025, following PRISMA guidelines. Due to heterogeneity in study designs and outcome measures, a narrative synthesis was performed. Studies evaluating CT in people with diabetes were included, with or without comparisons to non-diabetic individuals. After removing duplicates, 24 studies met the inclusion criteria. Data were extracted regarding study design, sample size, country of origin, type of diabetes, presence or severity of diabetic retinopathy (DR), OCT modality used, anatomical region examined, and CT outcomes. A total of 24 eligible observational studies were included. Choroidal thinning was reported in 11 studies, while 8 demonstrated thickening, often in early disease stages or in association with diabetic macular oedema (DMO). Five studies found no significant change in CT, although some reported reductions in the choroidal vascularity index despite stable thickness. Considerable variability was observed across study designs, imaging protocols, and study populations. Studies using swept-source OCT (SS-OCT) more consistently identified choroidal thinning compared to those using spectral-domain OCT (SD-OCT). Choroidal thickness appears reduced in people with diabetes, particularly with DR progression, supporting its potential role as a non-invasive biomarker of diabetic choroidopathy. However, the variability in findings underscores the need for standardised OCT imaging protocols and further longitudinal research to clarify the temporal relationship between CT alterations and diabetic disease progression.</p>