Nine other people, including fighters, were wounded in the explosion, the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
Turkish troops and allied rebel groups seized the Afrin region from Kurdish forces in March last year after a two-month air and ground offensive.
"The explosion is the result of a bomb that was placed in a bus in the centre of Afrin," Abdel Rahman said.
It was not immediately clear who was behind the blast, the second to rock Afrin since December 16 when a car bomb killed at least nine people, including five civilians, near a pro-Turkey rebel post in the city.
That explosion came after Turkish President Recep Tayyib Erdogan threatened to launch a new offensive against the Kurdish People's Protection Units (YPG) in Syria.
Turkey accuses the YPG of being "terrorists", but the Kurdish militia also forms the backbone of a US-backed alliance fighting the Islamic State group in Syria.