αβ and γδ T cell receptors (TCRs) are highly diverse antigen receptors that define two evolutionary conserved T cell lineages. We describe a population of γμTCRs found exclusively in non-eutherian mammals that consist of a two-domain (Vγ-Cγ) γ-chain paired to a three-domain (Vμ-Vμj-Cμ) μ-chain. γμTCRs were characterized by restricted diversity in the Vγ and Vμj domains and a highly diverse unpaired Vμ domain. Crystal structures of two distinct γμTCRs revealed the structural basis of the association of the γμTCR heterodimer. The γμ domain shared the characteristics of a single-domain antibody within which the hypervariable CDR3μ loop suggests a major antigen recognition determinant. We define here the molecular basis underpinning the assembly of a third TCR lineage, the γμTCR.