The court in Paris gave 32-year-old Moreau, a former fishermen and repeat offender, the maximum sentence after finding him guilty of association with a terrorist enterprise, ordering that he must serve at least two-thirds of it.
He had refused to attend court to hear the sentence.
Although he criticised IS for "brainwashing" and practising torture during his trial in December, the prosecutor warned that he was "extremely dangerous" and would probably revert to his "jihadist commitment" when released.
And Moreau told the court "If you give me a stiff sentence it will be more difficult to reintegrate me; I will take up arms again."
He went to join IS in January 2014 and remained in the region until June 2016, even opening a restaurant in its stronghold, Raqa, with his "war booty".
Brother of first French jihadist sentenced
His brother, Flavien Moreau, was the first jihadist to be convicted in France in November 2014 but received a lighter sentence having only spent a few weeks in IS-held territory.
Nicolas Moreau was born in South Korea and adopted by a French couple at the age of four.
He turned to crime after his adoptive parents divorced, converting to Islam while in prison.
Some 700 French nationals are currently fighting in IS's ranks in Iraq and Syria, mainly in Mosul, which Iraqi forces are recapturing from the Islamist movement.
On a visit to Iraq Monday, French President François Hollande declared that "Action against terrorism here in Iraq is also a way of preventing terrorist acts on our own soil."