The rebel al-Nusra Front said the man conducted the bombing on their behalf. It was one of four attacks carried out in the northern city of Idlib that day.
It is thought to be the first suicide attack by a US citizen in the conflict.
More than 100,000 people have been killed in the battle between forces loyal to Syrian President Bashar al-Assad and those opposed to his rule.
“I can confirm that this individual was a US citizen involved in a suicide bombing in Syria,” state department spokeswoman Jennifer Psaki said.
The man was believed to be Moner Mohammad Abu-Salha, she said.
The department was “concerned about the flow of foreign fighters in and out of Syria”, Jennifer Psaki added.
The al-Nusra Front, a militant Islamist group that has pledged allegiance to al-Qaeda, said the man used a truck carrying explosives to conduct the attack.
The Syrian Observatory for Human Rights said dozens of soldiers were killed by the series of bombings on Sunday, AFP news agency reported.
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