Forces loyal to Assad carried out the deadly April 4 attack, which could only have been ordered by the Syrian President and a few influential members of his inner circle, French intelligence assessed in a declassified report.
"The use of sarin is without question," France's Foreign Minister Jean-Marc Ayrault told reporters as he presented the results of the French investigation. "The responsibility of the Syrian regime is also without question."
Assad has denied carrying out the attack or any other chemical strikes on his people.
The sarin gas attack on Khan Sheikhoun in Syria's Idlib province killed more than 80 people, including dozens of children who died gasping for air. Many were still asleep when the airstrike hit the town in the early morning.
France's assessment is based on analysis of samples taken from the attack carried out by French experts. The Organisation for the Prohibition of Chemical Weapons (OPCW), as well as British and Turkish scientists also determined that sarin was used in the attack.
The Syrian government agreed to destroy its chemical weapons program in September 2013 after hundreds were killed in a sarin gas attack in East Ghouta outside of Damascus.
The French report said that the airstrike on Khan Sheikhoun showed similarities to a Syrian attack on Saraqib, also in Idlib, on April 29, 2013. This conclusion was based on analysis of an unexploded grenade which was used "with certainty" by Assad forces during the Saraqib attack, according to the report.
"The sarin present in the munitions used on 4 April was produced using the same manufacturing process as that used during the sarin attack perpetrated by the Syrian regime in Saraqib," the report read.
On the day of the Saraqib attack in 2013, a helicopter dropped three unidentified objects, emitting white smoke on neighborhoods to the west of the city, according to the report, which said that "only the Syrian armed forces had helicopters and could therefore be responsible for dropping these three objects."
The report also noted that al-Qaeda and ISIS in Syria do not have the capability to carry out such an attack.
Russia, Syria's ally, previously blamed rebels for the attack saying that a Syrian strike hit a warehouse where they were storing chemical weapons. Kremlin spokesman Dimitrije Peskov told reporters Wednesday that "the only way to restore the truth about what happened in Idlib is an impartial international investigation."