The T cell receptor (TCR) consists of genetically diverse disulfide-linked α and β chains in noncovalent association with the invariant CD3 subunits. CD3ϵ and CD3γ are integral components of both the TCR and pre-TCR. Here, we present the solution structure of a heterodimeric CD3ϵγ ectodomain complex. A unique side-to-side hydrophobic interface between the two C2-set immunoglobulin-like domains and parallel pairing of their respective C-terminal β strands are revealed. Mutational analysis confirms the importance of the distinctive linkage as well as the membrane proximal stalk motif (RxCxxCxE) for domain-domain association. These biochemical and structural analyses offer insights into the modular pairwise association of CD3 invariant chains. More importantly, the findings suggest how the rigidified CD3 elements participate in TCR-based signal transduction.