Brown seaweeds are known natural antioxidants and antiviral candidates. This study aimed to extract and characterize polysaccharides from Sargassum euryphyllum and Sargassum aquifolium brown macroalgal species and investigate their antiviral potential against avian infectious bronchitis virus (IBV). The chemical composition of the polysaccharides was characterized by Fourier transform infrared spectrometry, proton nuclear magnetic resonance, gel permeation chromatography, and high-performance liquid chromatography. Antioxidant activity was also determined as 2,2-diphenyl-1-picrylhydrazyl (DDPH) radical, hydrogen peroxide radical scavenging, and total antioxidant activity. Data revealed that S. aquifolium polysaccharides encompass significant ash (17.84% ± 0.69%), carbohydrates (57.43% ± 0.40%), sulfates (18.26% ± 0.52%), and uronic acids (45.40% ± 0.39%) content compared to S. euryphyllum polysaccharides. The molecular weight (MW) recorded 557 kDa for S. aquifolium polysaccharide with neutral sugars of mannose, galactose, and fructose, which constituted bulk molar ratios. A lower MW of 317 kDa was recorded for S. euryphyllum polysaccharide with mannose and xylose as the dominant neutral sugars. Both polysaccharides showed no cytotoxicity on chicken embryo kidney (CEK) cells. Potent antioxidant activity, measured as DPPH assay, for S. euryphyllum (59.15% ± 2.75%) and S. aquifolium (65.25% ± 2.89%) polysaccharides was established compared to ascorbic acid (67.33% ± 2.24%). The IBV titer on CEK cells in vitro showed a decreasing pattern from 5.4 to 4 log 10 median tissue culture infectious dose (TCID 50 )/ml with S. euryphyllum polysaccharide and from 4.5 to 2.2 log 10 TCID 50 /ml with S. aquifolium polysaccharide within 6 days post incubation time. A similar significant anti-IBV activity was estimated in vivo for both polysaccharides with a chorioallantoic membrane inoculation assay. Decreasing IBV titer suggested blocking of the virus entry, probably through disturbing its binding to the host cell receptors. The study recommended sulfated polysaccharides of S. aquifolium and S. euryphyllum seaweeds as potential antioxidants and antiviral natural resources.