Syrian President Bashar Al-Assad denied responsibility, but the alleged attack prompted a US-led airstrike on three suspected chemical weapons facilities before the investigation.
Maria Zakharova said on Sunday that the chemical attack could have been carried out by extremists or militants and Assad had destroyed his stockpiles.
She added: “Chemical weapons in Syria were destroyed.
“This is a proven fact and every time, when yet another settlement, another town is liberated by the Syrian army, the servicemen find depots with chemicals there.
“This has been proven by life, this has been proven by facts.”
The spokeswoman said there were lots of different chemical weapons in Syria and said the Russian side had sent a draft resolution to the UN Security Council to establish a mechanism to probe the origins of the weapons.
But she said the project was blocked by a group of the US-led states.
She also added that the US still possesses chemical weapons.
The rebel group in control of the Douma region at the time of the alleged chemical attack, Jaish al-Islam (Army of Islam), itself used chemical weapons against Kurds.
News outlet Kurdistan 24 said the group admitted using chemical weapons in a mainly Kurdish populated area in the city of Aleppo, northern Syria in April 2016.
In an official statement, Jaish al-Islam said: “One of the field commanders in Aleppo used weapons that he was not authorised to use in these kinds of confrontations.”
The group claimed the brigade commander was summoned to a military court, adding: “He has been referred to the Military Justice to receive the proper punishment.”
Both Syria and Russia claim the alleged attack in Douma could have been staged and the UK has even been implemented.
Russian Defence Ministry spokesman Major General Igor Konashenkov said: “We have evidence that proves Britain was directly involved in organising this provocation.”
Maj Gen Konashenkov said statements from witnesses at Douma’s hospital alleged people with video cameras rushed into the hospital, shouting that its patients were struck with chemical weapons and causing a mass panic.
He insisted doctors at the hospital declared none of the patients were hurt by chemicals.
British UN ambassador Karen Pierce rejected the allegations and branded them “grotesque”.
She said: “This is grotesque it is a blatant lie, it is the worst piece of fake news we’ve yet seen from the Russian propaganda machine.”