Nowadays, the development of new drugs only relies on a small number of molecules and 50% of all existent drugs are extracted or synthetically obtained. This work intends to evaluate the antibacterial potential of the ethanolic and polysaccharide extracts obtained from Grateloupia turuturu and to characterize the composition of the alga’s polysaccharides by FTIR-ATR. We used sequential extraction to obtain the extracts that were tested against S. aureus and E. coli. The ethanolic extracts in E. coli, at the highest concentration used (15 mg mL−1) showed 45.7% (Tetrasporophyte extract) and 55.1% (Carposporophyte extract) of growth reduction and in S. aureus 56.2% (T extract) and 51.8% (C extract). Polysaccharide extracts started showing significant reduction effect on E. coli and S. aureus growth at 7.5 mg mL−1 with a reduction of 54.9% and 39.5%, respectively. At 15 mg mL−1 the reduction observed was 88.5% and 85.4%. The FTIR-ATR allowed to characterize G. turuturu’s polysaccharides concluding that it is composed by a hybrid kappa/iota carrageenan with traces of agar, in both phases of the life cycle. This work allows us to conclude about the antibacterial properties of this alga and the compounds that might be behind this activity, showing that there’s a lot more than a small number of molecules that can be used as natural drugs.