The convoy contains food supplies, blankets and mattresses, family kits, hygiene kits, medicines and medical supplies for "hundreds of thousands of people desperately in need in the north of Syria", said the United Nations' regional humanitarian coordinator Nigel Fisher.
The shipments were widely seen as a possible step for more humanitarian access to the war-torn country's most desperate areas.
"It is the first time in three years of this brutal conflict that the UN has been able to carry aid across into Syria from Turkey," Fisher said.
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Non-governmental organisations have used the border crossing to ferry aid for some time.
The UN convoy left the Turkish town of Nusaybin, just across the border from the northeastern Syrian town of Qamishli whose population is mainly made up of Kurds, Christians and Arabs.
A Turkish official said the government gave the green light for the crossing of the convoy -- made up of 82 trucks -- via Turkey.
An estimated 146,000 people have been killed and millions displaced since the start of the uprising against Syria's President Bashar al-Assad three years ago.