Plant‐unique membrane receptor kinases with leucine‐rich repeat (LRR) extracellular domains are key regulators of development and immune responses. Here, the 1.55 Å resolution crystal structure of the immune receptor kinase SOBIR1 from Arabidopsis is presented. The ectodomain structure reveals the presence of five LRRs sandwiched between noncanonical capping domains. The disulfide‐bond‐stabilized N‐terminal cap harbours an unusual β‐hairpin structure. The C‐terminal cap features a highly positively charged linear motif which was found to be largely disordered in this structure. Size‐exclusion chromatography and right‐angle light‐scattering experiments suggest that SOBIR1 is a monomer in solution. The protruding β‐hairpin, a set of highly conserved basic residues at the inner surface of the SOBIR LRR domain and the presence of a genetic missense allele in LRR2 together suggest that the SOBIR1 ectodomain may mediate protein–protein interaction in plant immune signalling. The ectodomain structure of a novel plant membrane receptor kinase with unusual capping domains is reported.