There is an increasing need to find sustainable alternatives to conventional agrochemicals to reduce biotic stress in crops. One possible strategy is based on promoting the innate defences of plants by stimulating their immune system. The plant immune system relies on the perception of molecules, which trigger a cascade of biochemical responses known as pattern-triggered immunity (PTI). This study investigated the potential of marine macroalgal cell wall components to be perceived by plants, act as elicitors of plant immune responses and induce disease resistance. Cell walls of green, red, and brown algae species were chemically fractionated, and the research focused on testing their ability to induce immune responses in Arabidopsis thaliana. Different PTI hallmarks were tested, including H2O2 production, mitogen-activated protein kinases (MAPKs) phosphorylation, and defence gene expression analysis. The results showed that the CaCl2-extracted fraction was particularly efficacious in inducing H2O2 production. As the CaCl2 fraction of all phylogenetic groups also triggered additional immune responses, its ability to protect Arabidopsis against the bacterial pathogen Pseudomonas syringae was evaluated, confirming that certain CaCl2 fractions successfully provided resistance to the pathogen. The monosaccharide and glycosidic linkage analysis of these fractions pointed to some specific algal cell wall glycans (e.g. porphyrans and fucoidans) that could contribute to the immunostimulatory capacity, thereby paving the way for the identification of distinct structures with potential agrobiological applications.