CONTEXT: The soluble Fms-like tyrosine kinase-1/placental growth factor (sFlt-1/PlGF) ratio blood test can aid the prediction of preeclampsia (PE) in advance of a clinical diagnosis. Gestational diabetes mellitus (GDM) predisposes to PE, but may influence levels of these biomarkers independently of PE. OBJECTIVE: To determine whether the sFlt-1/PlGF ratio retains its clinical utility to predict PE in patients with GDM. METHODS: Retrospective analysis of sFlt-1/PlGF ratio and associated clinical outcome data acquired between September 2016 and September 2022, at the Royal Women's Hospital, Melbourne, Australia. 1416 sFlt-1/PlGF ratio results from pregnant women with and without GDM, and with and without PE were analysed. The main outcome measures were sFlt-1, PlGF and sFlt-1/PlGF ratio test results in relation to PE and GDM. RESULTS: There were no significant differences in the area under the curve (AUC), specificity, sensitivity, positive predictive value, or negative predictive value (NPV) between the PE + No GDM and PE + GDM groups in both the 0 to 7 and the 8 to 14 days before delivery categories. In the 0- to 7-day category the PE + No GDM group had an AUC of 0.82 (95% CI 0.79-0.87), sensitivity of 85.3% (95% CI 80.8-89.1%) and NPV of 92.7% (95% CI 90.5-94.5%), and the PE + GDM group had an AUC of 0.87 (95% CI 0.82-0.93), sensitivity of 89.4% (95% CI 76.9-96.5%), and NPV of 94.8% (95% CI 88.7-97.7%). CONCLUSION: The results of this investigation indicate that although GDM may lead to changes in the sFlt-1/PlGF ratio, the sFlt-1/PlGF ratio test retains its clinical utility in predicting PE onset.