The blast in the city near the Turkish border came four days ahead of the first local elections in Kurdish-controlled parts of Syria, opposed by Damascus and Ankara.
"Pro-regime militias... Booby-trapped a motorbike and placed it on a popular street in Qamishli, and then detonated it remotely," said Ali al-Hassan, a spokesman for the Asayesh security force.
The AFP correspondent saw people fleeing the site of the blast, which did not cause significant material damage.
The election set for Friday, more than a year after the Kurdish proclamation of a "federal region", was bitterly opposed by the Syrian regime and especially neighbouring Turkey.
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The Kurds deny any attempt to partition Syria, a country already torn apart by more than six years of war.
Regime forces withdrew from Kurdish-majority northern regions of Syria in 2012, months after the start of a rebellion against President Bashar Al-Assad.
Since then, the Kurds have strived to carve out an autonomous region.
They have become a key actor in the conflict and are spearheading the battle against the Islamic State group in the northern city of Raqa.
It is also opposed to a referendum on independence for Iraq's autonomous Kurdish region set for September 25.