Significance Misincorporation of dUTP into DNA is detrimental to eukaryotes, prokaryotes, and viruses. This study reveals the fate of uracilated HIV-1 DNA in human cells. Early stages of the viral life cycle are unaffected, but integration of uracilated viral DNA into the host genome is prevented. This effect is wholly dependent on the presence of UNG2, a nuclear enzyme that excises uracil from DNA. This study establishes that UNG2 is a restriction factor for uracilated HIV-1 DNA and explains why this pathway is not fully engaged in CD4+ T cells and macrophages.