Interleukin (IL)-11 is a member of the IL-6 family of pleiotropic cytokines that is characterised by shared use of the signal transducing receptor, GP130. Similar to IL-6, signalling by IL-11 is initiated by binding of soluble IL-11 to its membrane bound, specific receptor, IL-11Rα. This binary complex subsequently engages with GP130, inducing GP130 dimerisation and recruitment of JAK kinases, ultimately resulting in phosphorylation and activation of the transcription factor, Signal Transducer and Activator of Transcription (STAT)-3. As a result of the historical focus on the IL-6/STAT3 signalling axis, the IL-11 signalling machinery has remained relatively uncharacterised. No high resolution structural data have previously been reported for IL-11 or IL-11Rα and our understanding of the structure of the authentic signalling complex and the interactions between its components remains rudimentary. Here we report the crystal structure of human IL-11 and provide structural resolution of residues previously identified as important for IL-11 activity. Biophysical analysis indicates that IL-11 is a compact, stable, monomeric protein. While IL-11 is thought to signal via a complex analogous to that of IL-6, our comparisons show important differences between the two cytokines. Specifically, IL-11 shows a more elongated structure than IL-6 and lacks additional helical elements surrounding its central, 4-helix core. Detailed analysis of the GP130 binding regions suggest that IL-11 engages the first molecule of GP130 differently to IL-6. In addition to providing a structural platform for further study of IL-11, these data offer insight into the binding interactions of IL-11 with each of its receptors and the structural mechanisms underlying agonist and antagonist variants of the protein.