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 so-called Syrian Observatory for Human Rights said.
Such bombings are common in the town, which was held by Kurdish forces before Turkish troops and their proxies seized it last October.
The Turkish defense ministry blamed Sunday's attack on the US-backed Kurdish People's Protection Units (YPG), which still controls much of northeastern Syria, AFP reported.
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 organization PKK/YPG once more targeted innocent civilians," the ministry said on Twitter.
During its most recent incursion against the YPG last year, Ankara established a "safe zone" extending along 120 kilometers 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.