Background: Gestational diabetes mellitus (GDM) is a common complication of pregnancy that has a certain impact on maternal and infant health. The aim of this study was to investigate the diagnostic value of microRNA-204-5p (miR-204-5p) in GDM by analyzing its differential expression between GDM patients and healthy individuals and to assess its predictive potential for the occurrence of poor maternal and infant outcomes in GDM patients. Methods: GDM patients (107; GDM group) and healthy pregnant women (92; control group) were recruited for this prospective study. GDM patients meet the relevant criteria of the International Association of Diabetes and Pregnancy Study Groups (IADPSG), which are categorized into good and poor outcome groups on the basis of maternal and infant pregnancy outcomes. Serum miR-204-5p expression was quantified via the reverse transcription quantitative polymerase chain reaction (RT-qPCR). The diagnostic potential of miR-204-5p in GDM patients was analyzed by receiver operating characteristic (ROC) curves and further assessed for its ability to predict the occurrence of poor maternal and infant outcomes in GDM patients. The difference in clinical indicators between GDM patients and healthy pregnant women was determined through Student's t-test, and the clinical indicators of GDM patients in the good outcome group and the poor outcome group were evaluated. The potential of miR-204-5p as a risk factor for poor maternal and infant outcomes in GDM patients was evaluated by binary logistic analysis. Results: miR-204-5p expression was decreased in GDM patients compared to healthy pregnant women. The area under the curve (AUC) for distinguishing the healthy people from GDM patients was 0.918, with a sensitivity and specificity of 78.5% and 90.2%. Additionally, miR-204-5p expression in GDM patients in the poor outcome group was lower than that in the good outcome group. The AUC reflecting the potential of miR-204-5p in predicting poor maternal and infant outcomes in GDM patients was 0.855, with a sensitivity of 66.7% and a specificity of 90.5%. miR-204-5p is a risk factor for poor maternal and infant outcomes in GDM patients. Conclusions: Serum miR-204-5p expression is low in GDM and has a high diagnostic potential for predicting the occurrence of GDM and poor maternal and infant pregnancy outcomes. It is expected to be a biomarker for the risk of GDM.