Although it is known that the gut microbiota (GM) can be modulated by diet, the efficacy of specific dietary interventions in determining its composition and diversity in obese patients remains to be ascertained. The present work aims to evaluate the impact of a moderately hypocaloric Mediterranean diet on the GM of obese and overweight patients (OB). The GM of 23 OB patients (F/M= 20/3) was compared before (T0) and after 3 months (T3) of the nutritional intervention (NI). Fecal samples were analyzed by Illumina MiSeq sequencing of the 16S rRNA gene. At baseline, the GM characterization confirmed the typical obesity-associated dysbiosis. After 3 months of NI, patients presented a statistically significant reduction of the body weight and fat mass, along with changes in the relative abundance of many microbial patterns. In fact, we observed an increased abundance in several Bacteroidetes taxa (i.e. Sphingobacteriaceae, Sphingobacterium, Bacteroides spp., Prevotella stercorea) and depletion of many Firmicutes taxa (i.e. Lachnospiraceae members, Ruminococcaceae and Ruminococcus, Veillonellaceae, Catenibacterium, Megamonas). In addition, the phylum Proteobacteria showed an increased abundance, while the genus Sutterella, within the same phylum, decreased after the intervention. Metabolic pathways, predicted by bioinformatic analyses, showed a decrease in membrane transport and cell motility after NI. The present study extends our knowledge of the GM profiles in OB, highlighting the potential benefit of a moderate caloric restriction in counteracting the gut dysbiosis.