The Toll/interleukin-1 receptor (TIR) domains are key innate immune signalling modules. Here, we present the crystal structure of the TIR domain of human interleukin-1 receptor 10 (IL-1R10), also called interleukin 1 receptor accessory protein like 2. It is similar to that of IL-1R9 (IL-1RAPL1) but shows significant structural differences to those from Toll-like receptors (TLRs) and the adaptor proteins MyD88 adaptor-like protein (MAL) and MyD88. Interactions of TIR domains in their respective crystals and the higher-order assemblies (MAL and MyD88) reveal the presence of a common ‘BCD surface’, suggesting its functional significance. We also show that the TIR domains of IL-1R10 and IL-1R9 lack NADase activity, consistent with their structures. Our study provides a foundation for unravelling the functions of IL-1R9 and IL-1R10.