Yassin Sakkam was among 12 French citizens transferred to Iraqi authorities in January by a US-backed force fighting the jihadist group in neighbouring Syria.
"I admit to having sworn allegiance" to ISIS, he told the court, saying he was paid $70 (62 euros) a month.
Sakkam, who was dressed in a yellow prison uniform and sported a closely cropped goatee, said he regretted his decision to join IS, and asked to be pardoned.
Iraq has taken custody of thousands of jihadists in recent months after they were captured by the US-backed Syrian Democratic Forces during the battle to destroy the ISIS "caliphate".
They include hundreds of foreigners suspected of ISIS membership, raising the question of whether suspected ISIS jihadists should be tried in the region or repatriated.
France has long insisted its adult citizens captured in Iraq or Syria must face trial locally, while reiterating its opposition to capital punishment.
Iraqi law provides for the death penalty for anyone joining a "terrorist group" — even those who did not take up arms.
Sakkam, now aged 29, left France in late 2014 to fight for IS, posting pictures online of himself carrying arms and speaking to multiple media outlets about the group.
He became one of the most notorious jihadists from France, which has been seeking his arrest since 2016.
Kurdish authorities detained him in Syria in 2017.
His brother Karim carried out a suicide attack at the Iraqi-Jordanian border in 2015, according to the French Center for the Analysis of Terrorism.
'Inhumane treatment'
Also on Wednesday, an Iraqi court sentenced 24-year-old Tunisian Mohammed Berriri to death for joining ISIS, after a hearing lasting less than an hour.
Berriri, also dressed in a yellow prison uniform, admitted to joining IS, saying he thought it was "defending the weak", but said he now regretted doing so.
Sakkam and the six other French citizens handed death sentences in recent days have 30 days to appeal.
The remaining five French suspects face trial in the coming days.
Iraqi courts have dealt with a steady flow of jihadists — with 500 foreign men and women already found guilty of having joined IS. None of those sentenced to death have so far been executed.
The trials have been criticised by human rights groups, which say they often rely on evidence obtained through torture.
In a statement sent to AFP, a group representing the families of French jihadists asked the government in Paris to "do everything possible to stop this fatal chain of death sentences" and to try them "on our soil".
On Tuesday, Foreign Minister Jean-Yves Le Drian said France was stepping up efforts to stop Iraq executing those convicted.
France's rights ombudsman on Wednesday said the country must stop the "inhumane treatment" of children of jihadists stranded in Syria who are not being allowed to come to the country.
Meanwhile, Iraq repatriated to Turkey on Wednesday 188 children of Turks accused of belonging to the Islamic State group, a capital offence in Iraq, the judiciary said.
The move follows the repatriation to Tajikistan late last month of 84 children of nationals convicted by Iraqi courts of membership of IS or other jihadist groups.
That came after dozens of children were flown to Russia and after France and Germany repatriated the children of women nationals detained in Iraq.
Baghdad declared victory over IS in 2017 but the group's cross-border "caliphate" was only eliminated when US-backed fighters conquered its last scrap of territory in Syria in March.
Reporting by Sarah Benhaida
_(1).webp&w=3840&q=75)
