Various seaweed sulfated polysaccharides have been explored for antimicrobial application. This study aimed to evaluate the antibacterial activity of the native Gracilaria fisheri sulfated galactans (NSG) and depolymerized fractions against the marine pathogenic bacteria Vibrio parahaemolyticus and Vibrio harveyi. NSG was hydrolyzed in different concentrations of H₂O₂ to generate sulfated galactans degraded fractions (SGF). The molecular weight, structural characteristics, and physicochemical parameters of both NSG and SGF were determined. The results revealed that the high molecular weight NSG (228.33 kDa) was significantly degraded to SGFs of 115.76, 3.79, and 3.19 kDa by hydrolysis with 0.4, 2, and 10% H₂O₂, respectively. The Fourier transformed spectroscopy (FTIR) and ¹H− and ¹³C−Nuclear magnetic resonance (NMR) analyses demonstrated that the polysaccharide chain structure of SGFs was not affected by H₂O₂ degradation, but alterations were detected at the peak positions of some functional groups. In vitro study showed that SGFs significantly exerted a stronger antibacterial activity against V. parahaemolyticus and V. harveyi than NSG, which might be due to the low molecular weight and higher sulfation properties of SGF. SGF disrupted the bacterial cell membrane, resulting in leakage of intracellular biological components, and subsequently, cell death. Taken together, this study provides a basis for the exploitation and utilization of low-molecular-weight sulfated galactans from G. fisheri to prevent and control the shrimp pathogens.