Photographs and videos posted on social media showed a bearded and smiling Mr. al-Senussi, recognizable by his thick, curly black hair, being escorted off a helicopter.
Libyan officials later confirmed his arrival in the country.
Western and Libyan officials have linked Mr. al-Senussi to the 1988 bombing of PanAm Flight 103 over Lockerbie, Scotland, that killed 270 people, mostly Americans.
And Libyan officials have accused him of other crimes, including the massacre of more than 1,200 detainees at Tripoli’s notorious Abu Salim prison in 1996. It was not clear what charges the Libyan government would bring against Mr. al-Senussi.
Libya is obligated by a U.N. Security Council resolution to cooperate with the International Criminal Court, and Amnesty International said Libya must immediately hand over Mr. al-Senussi to the court.
“The ICC arrest warrant for al-Senussi remains in force, and Libya has an obligation to surrender him without delay to The Hague,” said Marek Marczynski, international justice research, policy and campaign manager at Amnesty International.
France also wants custody of Mr. al-Senussi, whom it charged in absentia for his role in a 1989 airline bombing that killed 54 French nationals.
Mr. al-Senussi was arrested in March after arriving on a flight from Morocco to the Mauritanian capital Nouakchott using a fake passport.
“If the extradition reports are confirmed, the decision to send him to Libya — with its weak justice system and inadequate fair-trial guarantees — will inevitably delay justice for victims and could lead to violations of al-Senussi’s rights to a fair trial,” Mr. Marczynski said.
Human Rights Watch, a New York-based group, said the Libyan government must cooperate with the ICC.