Seaweed-derived polysaccharides have attracted more attention due to their possible prebiotic qualities and positive health impacts. The extraction, identification, and measurement of polysaccharides from particular seaweed species are the main objectives of this work. An in vitro evaluation of the polysaccharide’s effects on the human gut microbiota is conducted after they have been extracted, characterized, and quantified from a few chosen seaweed species. Ethanol precipitation and dialysis were used to purify the polysaccharides after they were isolated by the use of hot water extraction. A structural study was performed to determine the composition of monosaccharides using FTIR (Fourier-transform infrared spectroscopy) and HPLC (High-performance liquid chromatography). Human fecal inocula were used in in vitro fermentation experiments to evaluate changes in the structure of the microbial community by the use of 16S rRNA gene sequencing. The results showed that seaweed polysaccharides, especially sulfated galactans and fucoidans, decreased the number of calculating pathogens while highly improving the growth of Lactobacillus and Bifidobacterium, which are two types of healthy gut bacteria. According to these results, seaweed polysaccharides may act as functional prebiotics that support a more wholesome gut flora and provide a basis for dietary therapies targeted at enhancing metabolic and gastrointestinal health.