The biological activities of water-soluble sulfated polysaccharides of green algae (ulvans) have been explored for use as bioactive molecules for the benefit of human and animal health. A purified ulvan fraction was prepared from the green algae Ulva armoricana harvested in the Brittany (France) and tested for its capacity to stimulate the immune response of the gut using an in vitro system of porcine intestinal epithelial (IPEC-1) cells. RT-qPCR and ELISA analyses showed a significant increase in the mRNA and protein expression of cytokines such as CCL20, IL8, and TNFα. Using human embryonic kidney (HEK) 293 reporter cell lines for pattern recognition receptors, ulvan was found to primarily stimulate TLR4. We also examined the effect of the ulvan fraction on different signalling pathways involved in the activation of cytokine gene expression. Western blot analyses of ulvan-treated HEK293-TLR4 cells showed an increase in the phosphorylation of Akt and the p65 subunit of nuclear factor-κB. Inhibition of Akt phosphorylation with the specific inhibitor abrogated the ulvan-mediated enhancement of IL-8 secretion. The overall results showed that ulvan is an immunostimulatory compound by itself, and furthermore, it could be used to effectively complex and deliver TLR ligands to relevant immune cells in vaccination strategies.