HIV-1 and other lentiviruses have the unique property among retroviruses to replicate in nondividing cells. This property relies on the use of a nuclear import pathway enabling the viral DNA to cross the nuclear membrane of the host cell. In HIV-1 reverse transcription, a central strand displacement event consecutive to central initiation and termination of plus strand synthesis creates a plus strand overlap: the central DNA flap. We show here that the central DNA flap acts as a cis-determinant of HIV-1 DNA nuclear import. Wild-type viral linear DNA is almost entirely imported into the nucleus where it integrates or circularizes. In contrast, mutant viral DNA, which lacks the DNA flap, accumulates in infected cells as unintegrated linear DNA, at the vicinity of the nuclear membrane. Consistently, HIV-1 vectors devoid of DNA flap exhibit a strong defect of nuclear import, which can be corrected to wild-type levels by reinsertion of the DNA flap sequence.