Whilst αβ T cell recognise MHC-Ag complexes for immunity, very little is known about the Ag-presentation elements of γδ T cells. Recently, butyrophilin (BTN) molecules have emerged as key regulators of γδ T cell immunity, however, the mechanism by which they activate the γδTCR is unknown. In humans, most circulating γδ T cells are phosphoantigen (pAg)-reactive Vγ9Vδ2+ T cells, which play a critical role in immunity to most bacterial and apicomplexan parasites infections as well as cancer. To explore how BTNs co-ordinate γδ T cell immunity, we solved the crystal structure of Vγ9Vδ2+ TCR in complex with BTN member 2A1 (BTN2A1), revealing that BTN2A1 engages the side of the γδTCR. Intriguingly, we also found that a second ligand, namely BTN3A1, can bind the exposed apical surface of Vγ9Vδ2+ TCR alongside BTN2A1. However, BTN3A1 binding only occurred following cross-linking with an agonist anti-BTN3A1 mAb, or alternatively, following mutation of a negative regulatory residue, Lys53 of the TCR δ-chain. Unexpectedly, BTN2A1 and BTN3A1 ectodomains were also shown to interact directly with each other in cis, forming heteromers on the surface of Ag-presenting cells, and that this BTN2A1–BTN3A1 interaction depends upon the same epitopes that BTN2A1 and BTN3A1 each use to engage γδTCR. We propose that this heteromer represents a ‘closed state’, which impairs the ability of γδTCR to bind the BTN complex. Indeed, either forced separation or locking together of BTN2A1 and BTN3A1 resulted in enhanced or abrogated γδTCR reactivity, respectively. Finally, we demonstrate that pAg can modulate γδTCR affinity for the BTN complex, suggesting that pAg-sequestration can lead to a conformational change in the BTN ectodomains enabling γδTCR to bind. Our findings reveal a new paradigm in immune activation, whereby γδ T cells recognise BTN complexes following conversion of BTN epitopes from an closed-state into a permissive-state.