Objective: To summarize evidence from longitudinal observational studies to determine whether diabetes (types 1 and 2) is associated with the course of symptoms in people with frozen shoulder. Data Sources: A systematic literature search of 11 bibliographic databases (published through June 2021), reference screening, and emailing professional contacts. Study Selection: Studies were selected if they had a longitudinal observational design that included people diagnosed with frozen shoulder at baseline and compared outcomes at follow-up (>2wk) among those with and without diabetes at baseline. Data Extraction: Data extraction was completed by 1 reviewer using a predefined extraction sheet and was checked by another reviewer. Two reviewers independently judged risk of bias using the Quality in Prognostic Factor Studies tool. Data Synthesis: A narrative synthesis, including inspection of forest plots and use of the prognostic factor Grading of Recommendations, Assessment, Development and Evaluations framework. Twenty-eight studies satisfied the inclusion criteria. Seven studies were judged to be at a moderate risk of bias and 21 at a high risk of bias. Diabetes was associated with worse multidimensional clinical scores (moderate certainty in evidence), worse pain (low certainty in evidence), and worse range of motion (very low certainty in evidence). Conclusions: This review provides preliminary evidence to suggest that people with diabetes may experience worse outcomes from frozen shoulder than those without diabetes. If high-quality studies can confirm the findings of this review, then clinicians should monitor patients with frozen shoulder with diabetes more closely and offer further treatment if pain or lack of function persists long-term.