The Syrian Observatory for Human Rights said it was unclear if the strike on the rebel-held town in northwest Syria's Idlib province was carried out by Syrian planes or those of government ally Russia.
The death was the first in the town since a suspected chemical weapons attack on Tuesday that killed 87 civilians, including 31 children, and left hundreds suffering symptoms including convulsions, vomiting and foaming at the mouth.
Much of the international community pointed the finger at President Bashar al-Assad's government for the attack, though it denied any responsibility.
US President Donald Trump ordered the first direct US military action against Assad's government in response to the attack, launching missiles against an air base in central Syria.
Khan Sheikhun has been hit several times since the Tuesday attack, including in a strike in the hours afterwards on a hospital treating victims.
Idlib province is largely held by an alliance of rebels, including a former Al-Qaeda affiliate, and is regularly targeted in Syrian and Russian air strikes.
The US-led coalition fighting the Islamic State jihadist group has also carried out rare strikes in the province.
More than 320,000 people have been killed in Syria since its conflict began in March 2011 with anti-government demonstrations.
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