Douma was held by rebels at the time of the incident, which came during a major offensive by the pro-Assad forces to retake the town near the capital Damascus.
Emergency workers said at the time that they had treated people suffering breathing problems, foaming at the mouth and other symptoms.
OPCW inspectors visited the scene after delays and determined that chlorine was used, however they did not have the remit at the time to say who they believed was behind the attack.
But thanks to new rules, which Syria and Russia have opposed, the watchdog is now able to point the finger of blame, and in this case has done so at Damascus.
"There are reasonable grounds to believe that the Syrian Arab Air Forces were the perpetrators of the chemical weapons attack... in Douma," it said.
Syria has accused rebels and emergency workers of staging the attack it by bringing in dead bodies and filming them, or alternatively arguing that an Islamist chemical weapons factory was hit.
But the OPCW said its team "thoroughly pursued lines of inquiry and scenarios suggested by Syrian authorities and other state parties, but was unable to obtain any concrete information supporting them."
It also said it "regrets" that Syria refused to allow it further access to the site to investigate.
The report also dismisses claims by former inspectors who alleged that the watchdog had altered original findings in 2018 to make the evidence of a chemical attack seem more conclusive.
It added that the basis of "reasonable grounds' was the "standard of proof consistently adopted by international fact-finding bodies and commissions of inquiry."
Damascus denies the use of chemical weapons and insists it has handed over its stockpiles under a 2013 agreement, prompted by a suspected sarin gas attack that killed 1,400 in the Damascus suburb of Ghouta.
