A UN spokesman confirmed that Syria had sent accession documents to the world body, which is guarantor of the 1993 convention banning the production and stockpiling of the arms.
Syria's UN envoy said joining the convention was the end of a "chapter" in the Syria crisis.
"In the past few hours we have received a document from the government of Syria," said UN spokesman Farhan Haq, adding that it was "an accession document."
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Syria had been one of seven UN members that have refused to join the 1993 convention.
But President Bashar al-Assad's government announced it would sign up as part of a Russian plan to put his country's chemical arsenal under international control.
The United States and other western nations accuse Assad's government of launching a sarin gas attack on August 21 near Damascus in which hundreds died and Washington has threatened a punitive military strike.
The UN spokesman said it could take a few days to complete the accession process.
Syria's UN ambassador Bashar Jaafari said his government now considered itself a full member of the convention.
"With this, the chapter of the so-called chemical weapons should be ended," Jaafari told reporters.
"The chemical weapons in Syria are a mere deterrence against the Israeli nuclear arsenal," he added.
Jaafari said he expected a UN report on the August 21 attack -- which his government blames on opposition rebels -- to be handed to UN leader Ban Ki-moon early next week.
"We have nothing to hide," the ambassador said, while adding that Syria does not want "any partial report, any politicized report, any manipulated report."
Under the 1993 convention, Syria will have to destroy any chemical arms it possesses.
