Background: Some data indicate that Mediterranean diet could benefit children with Type 1 diabetes by positively affecting blood glucose control and gut microbiome. Its effects on adults are unknown. Aims: To investigate the links between Mediterranean diet, blood glucose control, and gut microbiome in adults with T1D, also exploring the feasibility of using gut microbiome profiles as predictive markers for clinical parameters and dietary patterns. Materials and methods: In a cross-sectional study on 253 individuals with T1D, 124 men/129 women, age 38.7±13.3 years, BMI 25.7±4.4 kg/m2, and glycated hemoglobin (HbA1c) 7.5±0.9%, diet was assessed by EPIC (European Prospective Investigation into Cancer and Nutrition) questionnaire, adherence to the Mediterranean diet was evaluated with the relative Mediterranean diet score (r-MED), and gut microbiome was analyzed from fecal samples through shotgun metagenomic sequencing (n=103). Results: Mediterranean diet adherence inversely correlated with HbA1c levels (r=-0.17, p=0.02). At species-level microbiome taxonomy, several taxa showed a significantly different abundance between high and low adherence to Mediterranean diet. Specifically, high adherence was associated with an enrichment in beneficial species including Faecalibacterium prausnitzii and Ligilactobacillus salivarius. Conversely, low adherence corresponded to an overrepresentation of potential deleterious species such as Ruthenibacterium lactatiformans and Eggerthellaceae bacterium. A machine learning approach was used to build regression models assessing predictability of clinical parameters and dietary patterns from microbiome composition. Microbiome significantly predicted several variables including HbA1c (R2=0.3, p<0.001) and Mediterranean diet adherence (R2=0.35, p<0.001). Conclusion: Our results highlight the possible effects of Mediterranean diet adherence on blood glucose control and gut microbiome composition in adults with T1D.