In a short statement, the Lebanese army said Majid al-Majid "died this morning while undergoing treatment at the central military hospital after his health deteriorated." It did not elaborate.
Earlier, a Lebanese army general told The Associated Press that al-Majid died after suffering kidney failure. He was speaking on condition of anonymity in line with regulations. State-run National News Agency said al-Majid died "after his health conditions deteriorated."
Al-Majid, a Saudi citizen, was detained in Lebanon late last month and had been held at a secret location.
He was the purported commander of the Abdullah Azzam Brigades — a Sunni militant group with al-Qaida links — and one of the 85 most-wanted individuals in his native Saudi Arabia.
The U.S. State Department designated his group a foreign terrorist organization in 2012, freezing any assets it holds in the United States and banning Americans from doing business with the group.
The brigades have claimed responsibility for attacks throughout the region, including the 2010 bombing of a Japanese oil tanker in the Persian Gulf and several rocket strikes from Lebanon into Israel.
The most recent attack claimed by the group was the double suicide bombing in November outside the Iranian Embassy in Beirut that killed at least 23 people and wounded dozens.
Reports first surfaced about his arrest in Lebanon early this week. Security officials eventually confirmed that they had a suspect in custody, but said they were not certain of his identity.
On Friday, the Lebanese confirmed his identity, following a DNA test.
Al-Majid was believed to have serious kidney problems that require dialysis. He was an important figure, and the Abdullah Azzam Brigades grew from a relatively small outfit to a larger terror group since he took over in mid-2012, after the organization's previous leader, Saleh al-Qarawi, was gravely wounded in Pakistan.
According to Lebanese newspapers, al-Majid was detained during the last week of December while on his way from Beirut to the eastern Bekaa Valley that borders Syria. The reports said that he was captured while in an ambulance after he had undergone dialysis at a hospital in Beirut.
In the spring of 2013, after the Lebanese Shiite Hezbollah group announced that it was fighting alongside Syrian President Bashar Assad's troops against the Syrian rebels, the Abdullah Azzam Brigades began to target Hezbollah as well — and by extension, their Iranian patrons.
On Friday, families of those killed in the Iranian embassy bombing demanded that al-Majid, who had not been charged in the attack, be tried in Lebanon and not be sent to his homeland.