The letter that Syria will abide by the 1992 treaty to ban chemical weapons was received by U.N. Secretary-General Ban Ki-moon, Ban's spokesman said.
Earlier, Assad had been quoted as saying during an interview with a Russian TV that Syria intends to join the treaty.
Ban's spokesman in a statement said Assad "has signed the legislative decree providing for the accession of Syria" to the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction.
The spokesman said the letter expressed Syrian authorities' "commitment to observe the obligations entailed by the Convention even before its entry into force for Syria" and that the Secretary-General welcomed the development.
Ban also expressed hopes the current talks in Geneva would lead to speedy agreement which would be endorsed and assisted by the international community.
U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have been meeting in Geneva for talks on Russia's proposal for Syria to surrender its chemical weapons and place them under international control.
Separately, the United Nations said evidence collected by a U.N. team on the use of chemical weapons in Syria on Au. 21 was being examined by laboratories in Europe and that the results would be shared with Ban, who will then share them with the 15-member Security Council and other U.N. member states.