The blast from an explosive-rigged motorbike ripped through a vegetable market in the volatile town of Ras Al Ain, wounding at least 19 people, the Syrian Observatory for Human Rights said.
Such bombings are common in the town, which was held by Kurdish forces before Turkish troops and their Syria proxies seized it last October.
The Turkish defence ministry blamed Sunday's attack on the US-backed Kurdish People's Protection Units (YPG), which still controls much of north-eastern Syria.
Despite the key role the YPG played in the US-led campaign to drive the Daesh group out of northern and eastern Syria, Ankara has launched repeated incursions against the group, which it regards as an offshoot of the Kurdistan Workers' Party (PKK) that has waged a deadly insurgency in southeastern Turkey since 1984.
"The terror organisation PKK/YPG once more targeted innocent civilians," the ministry said on T witter.
During its most recent incursion against the YPG last year, Ankara established a "safe zone" extending along 120 kilometres of the border and including the town of Ras Al Ain.
The area is a frequent target for bombings and other attacks.
Last week, a car bomb killed four people and wounded 10.