Amid high security French national Mehdi Nemmouche, 33, went on trial on 10 January, accused of being the gunman.
Nemmouche, who had returned to Europe after fighting with jihadist forces in Syria, is on trial along with Nacer Bendrer, 30, who allegedly supplied him with a revolver and an assault rifle.
On 26 May 2014, he allegedly shot an Israeli couple, a Belgian museum worker and a French volunteer during an attack which lasted only 82 seconds.
"As you have seen, he is serene, calm, and he will decide for the moment when he will speak," his lawyer, Henri Laquay, told reporters.
The first two days of the trial will be taken up by reading out the 200 pages of the indictment.
Both men face life sentences if convicted of the charge of "terrorist murder".
More than 100 witnesses are due to testify.
Nemmouche's family is of Algerian origin but he grew up in the industrial city of Roubaix, near the Belgian border.
— Méabh Mc Mahon (@Brusselsness) 10 January 2019
He was arrested in Marseille six days after the attack, after arriving on a coach from Brussels.
Prosecutors claim he was still carrying the weapons used in the massacre.
They say he fought with a jihadist faction in Syria from 2013 to 2014, where he met Najim Laachraoui, a member of the gang which went on to carry out suicide bombings in Brussels that killed 32 people in March 2016.