Obesity is a growing public health problem. In recent decades, scientific evidence has linked gut microbiota to obesity. This systematic review summarizes current knowledge on the composition and functional differences in gut microbiota between individuals with obesity and those with normal weight. Following PRISMA 2020 recommendations, studies published in adult populations between January 2014 and May 2024 were reviewed. PubMed, Web of Science, and Scopus databases were searched for observational studies that had used advanced sequencing methods, such as 16S rRNA and shotgun metagenomics, to assess gut microbiota. The quality of these studies was also analyzed using the Newcastle-Ottawa scale. Our review of 16 studies shows a reduction in microbial diversity in individuals with obesity. In addition, a higher relative abundance of the phylum Firmicutes, the families Enterobacteriaceae, Gemellaceae, Prevotellaceae, Streptococcaceae and Veillonellaceae, as well as the genera Blautia, Butyricimonas, Collinsella, Megamonas, and Streptococcus, while beneficial bacteria such as the families Porphyromonadaceae and Rikenellaceae, and the genera Bifidobacterium spp. and Faecalibacterium prausnitzii, were depleted. Functional analysis showed a tendency to an increase in metabolic pathways associated with carbohydrate and lipid metabolism, with reduced pathways related to short-chain fatty acid production. Obesity is associated with altered gut microbiota composition and function. However, the variability across studies regarding population characteristics, dietary pattern, and sequencing techniques limits the comparability of findings. Future research should prioritize standardized methodologies and confounding factors to elucidate the role of the gut microbiome in obesity.