Discovery of the jasmonate ZIM-domain (JAZ) repressors defined the core jasmonate (JA) signalling module as COI1–JAZ–MYC2, and allowed a full view of the JA signalling pathway from hormone perception to transcriptional reprogramming. JAZ proteins are repressors of MYC2 and targets of SCFCOI1, which is the likely jasmonate receptor. Upon hormone perception, JAZ repressors are degraded by the proteasome releasing MYC2 and allowing the activation of JA responses. All members of the JAZ family share two conserved domains, the Jas motif, required for JAZ interactions with MYC2 and COI1, and the ZIM domain, the function of which is so far unknown. Here, we show that the ZIM domain acts as a protein–protein interaction domain mediating homo- and heteromeric interactions between JAZ proteins. These JAZ–JAZ interactions are independent of the presence of the hormone. The observation that only a few members of the JAZ family form homo- and heteromers may suggest the relevance of these proteins in the regulation of JA signalling. Interestingly, the JAZ3ΔJas protein interacts with several JAZ proteins, providing new clues to understanding the dominant JA insensitivity promoted by truncated JAZΔJas proteins. We also provide evidence that the Jas motif mediates the hormone-dependent interaction between Arabidopsis JAZ3 and COI1, and further confirm that the Jas motif is required and sufficient for Arabidopsis JAZ3–MYC2 interaction. Finally, we show that interaction with MYC2 is a common feature of the JAZ family, as most JAZ proteins can bind MYC2 in pull-down and yeast two-hybrid assays.