Abstract Purpose To investigate the relationships between the habitual diet, the protein to fiber ratio (P/F), and the gut microbiome in one Italian and one Dutch cohort of healthy subjects consuming an omnivore diet. Methods The Italian cohort included 19 males (M_IT, BMI 25.2 ± 0.72 kg/m 2 , age 25.4 ± 0.96 years) and 20 females (F_IT, BMI 23.9 ± 0.81 kg/m 2 , age 23.8 ± 0.54 years); the Dutch cohort included 30 females (F_NL, BMI: 23.9 ± 0.81 kg/m 2 , age: 23.8 ± 0.54 years). Individual diets were recorded through Food Frequency Questionnaires and analyzed to assess the nutrient composition. Gut microbiome was assessed in fecal samples. Results M_IT consumed higher levels of proteins than F_NL and F_IT, whereas dietary fiber intake did not differ among groups. Data showed that consumption of plant protein to animal protein (PP/AP) and PP to total proteins ratio can determine a differentiation of F_NL more than the absolute amount of dietary fiber. Conversely, the protein to fiber (P/F) and AP to total proteins better characterized M_IT. M_IT harbored the highest abundance of proteolytic microorganisms and the lowest microbial gene richness. Conversely, F_NL had more fiber-degrading microorganisms like Bacteroides thetaiotaomicron , Bacteroides xylanisolvens, Roseburia sp., Coprococcus eutactus and Parabacteroides along with the highest number of genes encoding carbohydrate-active enzymes and gene richness. It was predicted that by each unit decrease in the P/F a 3% increase in gene richness occurred. Conclusion Study findings suggested that dietary P/F, rather than the absolute amount of dietary fiber, could contribute to the shaping of the microbiome towards a more proteolytic or fiber-degrading gut ecosystem. ClinicalTrials.gov Identifier NCT04205045—01-10-2018, retrospectively registered. Dutch Trial Register NTR7531—05-10-2018.