The Carma1–Bcl10–Malt1 (CBM) complex bridges T‐cell receptor (TCR) signalling to the canonical IκB kinase (IKK)/NF‐κB pathway. NF‐κB activation is triggered by PKCθ‐dependent phosphorylation of Carma1 after TCR/CD28 co‐stimulation. PKCθ‐phosphorylated Carma1 was suggested to function as a molecular scaffold that recruits preassembled Bcl10–Malt1 complexes to the membrane. We have identified the serine–threonine protein phosphatase PP2A regulatory subunit Aα (PPP2R1A) as a novel interaction partner of Carma1. PPP2R1A is associated with Carma1 in resting as well as activated T cells in the context of the active CBM complex. By siRNA‐mediated knockdown and in vitro dephosphorylation, we demonstrate that PP2A removes PKCθ‐dependent phosphorylation of Ser645 in Carma1, and show that maintenance of this phosphorylation is correlated with increased T‐cell activation. As a result of PP2A inactivation, we find that enhanced Carma1 S645 phosphorylation augments CBM complex formation, NF‐κB activation and IL‐2 or IFN‐γ production after stimulation of Jurkat T cells or murine Th1 cells. Thus, our data define PP2A‐mediated dephosphorylation of Carma1 as a critical step to limit T‐cell activation and effector cytokine production.