Significance T cell receptors (TCRs) on the surface of T cells mediate recognition of antigen. Since each new T cell carries an individual clonal TCR, monitoring of TCR repertoires reflects how T cells react and proliferate in response to environmental cues in the developing immune system of neonates and children. γδ T cells appear early during ontogeny and are important for immune surveillance. Here, we longitudinally analyze γδ T cells in neonates and show an immediate polyclonal expansion of phosphoantigen-reactive Vγ9Vδ2 ⁺ T cells after birth. We also observed differences in γδ TCR repertoires of children from Europe and Africa. Our study highlights the importance of γδ T cells in the neonatal immune system and their prompt expansion directly after birth.