The paracaspase mucosa‐associated lymphoid tissue 1 (MALT1) is central to lymphocyte activation and lymphomagenesis. MALT1 mediates antigen receptor signalling to NF‐κB by acting as a scaffold protein. Furthermore, MALT1 has proteolytic activity that contributes to optimal NF‐κB activation by cleaving the NF‐κB inhibitor A20. Whether MALT1 protease activity is involved in other signalling pathways, and the identity of the relevant substrates, is unknown. Here, we show that T‐cell receptors (TCR) activation, as well as overexpression of the oncogenic API2–MALT1 fusion protein, results in proteolytic inactivation of CYLD by MALT1, which is specifically required for c‐jun N‐terminal kinase (JNK) activation and the inducible expression of a subset of genes. These results indicate a novel role for MALT1 proteolytic activity in TCR‐induced JNK activation and reveal CYLD cleavage as the underlying mechanism.