DNA double-strand breaks represent one of the most severe forms of DNA damage in mammalian cells. One pathway for repairing these breaks occurs via nonhomologous end-joining (NHEJ) and depends on XRCC4, LigaseIV, and Cernunnos, also called XLF. Although XLF stimulates XRCC4/LigaseIV to ligate mismatched and noncohesive DNA ends, the mechanistic basis for this function remains unclear. Here we report the structure of a partially functional 224 residue N-terminal fragment of human XLF. Despite only weak sequence similarity, XLF(1-170) shares structural homology with XRCC4(1-159). However, unlike the highly extended 130 A helical domain observed in XRCC4, XLF adopts a more compact, folded helical C-terminal region involving two turns and a twist, wrapping back to the structurally conserved N terminus. Mutational analysis of XLF and XRCC4 reveals a potential interaction interface, suggesting a mechanism for how XLF stimulates the ligation of mismatched ends.