An aqueous marine-sulfated polysaccharide (MSP) extract, prepared from the green macroalga Ulva armoricana, was tested as an antibacterial compound against 42 bacterial strains and isolates found in livestock animals. Both Gram-positive and Gram-negative bacteria growths were affected. The most susceptible pathogens were Pasteurella multocida, Mannheimia haemolytica, Erysipelothrix rhusiopathiae, Staphylococcus aureus, and Streptococcus suis, with a minimum inhibitory concentration (MIC) ranging from 0.16 to 6.25 mg mL−1. Enterococcus cecorum, Streptococcus dysgalactiae, Corynebacterium, Trueperella pyogenes, and Bordetella bronchiseptica strains were also susceptive to MSP with a complete inhibition recorded at MIC values of 25 and 50 mg mL−1. The stimulation of the immune response mediators of the host’s gut with the extract was evaluated using an in vitro system of differentiated porcine intestinal epithelial cells (IPEC-1). RT-qPCR analysis showed a significant increase of mRNA expression of cytokines such as IL1α, IL1β, L6, IL8, TNFα, and TGFβ as well as the chemokine CCL20. The extract also significantly induced the expression of PPARγ, a ligand-activated transcription factor, and TLR2 receptor.