The move comes days after France said a "disciplinary procedure" for withdrawing the award was under way.
France recently joined the US and Britain in bombing Syrian targets over an alleged chemical weapons attack.
The award was returned to France via the Romanian embassy in Damascus.
President Assad was decorated with the highest class of the award, the Grand Croix, in 2001 after he took power following the death of his father.
"The ministry of foreign affairs... has returned to the French republic... the decoration of the Grand Croix of the Legion d'honneur awarded to President Assad," the Syrian foreign ministry said in a statement.
"It is no honour for President Assad to wear a decoration attributed by a slave country and follower of the United States that supports terrorists," it added.
About 3,000 people every year are awarded the Legion d'honneur for "services rendered to France" or for defending human rights, press freedom or similar causes.
The US, UK and France bombed several Syrian government sites on Saturday in retaliation for an alleged chemical weapons attack on Douma, the last rebel-held town in the Eastern Ghouta region outside Damascus.
More than 40 people were killed in the 7 April attack, according to opposition activists, rescue workers and medics.
The Syrian government denies using chemical weapons and says the attack was fabricated.
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