The visit to the revered southern city capped Le Drian's packed two-day trip to Iraq, during which he announced new support for the country's reconstruction efforts.
France would issue a new one-billion-euro loan over the next four years to help Iraq rebuild after the gruelling fight against the Islamic State group, he said.
And he declared that French businesses were "absolutely available to contribute to Iraq's reconstruction."
Many foreign firms pulled out of Iraq when IS seized swathes of the country in 2014.
Paris granted Baghdad a 430-million-euro loan in 2017 to help implement reforms and improve public services, whose dire state sparked protests last summer.
Le Drian landed in Baghdad on Monday, where he met with Prime Minister Adel Abdel-Mahdi and President Barham Saleh before travelling to the Iraqi Kurdish regional capital Arbil.
On Tuesday afternoon in Najaf he visited the tomb of Imam Ali, son-in-law of the Prophet Mohammed and a founding figure of Shiite Islam.
He also met with Ayatollah Mohammad Said al-Hakim, one of Iraq's top four Shiite clerics.