Peroxisome proliferator activated receptor gamma (PPAR gamma) is a nuclear receptor and target for antidiabetics that increase insulin sensitivity. Owing to the side effects of PPAR gamma full agonists, research has recently focused on non-activating ligands of PPAR gamma, which increase insulin sensitivity with decreased side effects. Here, we present the crystal structures of inverse agonist SR10171 and a chemically related antagonist SR11023 bound to the PPAR gamma ligand-binding domain, revealing an allosteric switch in the activation helix, helix 12 (H12), forming an antagonist conformation in the receptor. H12 interacts with the antagonists to become fixed in an alternative location. Native mass spectrometry indicates that this prevents contacts with coactivator peptides and allows binding of corepressor peptides. Antagonists of related nuclear receptors act to sterically prevent the active configuration of H12, whereas these antagonists of PPAR gamma alternatively trap H12 in an inactive configuration, which we have termed the tumble and trap mechanism.