Immune responses in plants are triggered by molecular patterns or elicitors, recognized by plant pattern recognition receptors. Such molecular patterns are the consequence of host-pathogen interactions, and the response cascade activated after their perception is known as pattern-triggered immunity (PTI). Glucans have emerged as key players in PTI, but the ability of certain glucans to stimulate defensive responses in plants remains understudied. This work focused on identifying novel glucan oligosaccharides as molecular patterns. The ability of various microorganism-derived glucans to trigger PTI responses was tested, revealing that specific microbial-derived molecules, such as short linear β-1,2-glucans, trigger this response in plants by increasing the production of reactive oxygen species (ROS), mitogen-activated protein kinase phosphorylation, and differential expression of defence-related genes in Arabidopsis thaliana. Pre-treatments with β-1,2-glucan trisaccharide (B2G3) improved Arabidopsis defence against bacterial and fungal infections in a hypersusceptible genotype. The knowledge generated was then transferred to the monocotyledonous model species maize and wheat, demonstrating that these plants also respond to β-1,2-glucans, with increased ROS production and improved protection against fungal infections following B2G3 pre-treatments. In summary, as with other β-glucans, plants perceive β-1,2-glucans as warning signals which stimulate defence responses against phytopathogens.