Levi Jonathan Shirley, a former US marine who went to Syria to fight against Islamic State, was killed last week in a battle against the extremist militia, local Kurds and a monitoring group reported on Friday.
Shirley was killed in the northern Syrian city of Minbij while battling along with the People's Protection Units (YPG), a Kurdish militia, the Syrian Observatory for Human Rights said.
Shirley, who was almost 25, came from Nevada and joined the YPG in February 2015.
His mother, Susan Shirley, confirmed his death to the US-based broadcaster NBC News.
The YPG said that Shirley, who had the Kurdish nom de guerre Agir Servan, had been killed on July 14.
"Hevale (Friend) Agir was known for his discipline and sense of responsibility. His style and personality were a source of strength, motivation, and morale for his friends," the YPG added in a statement, according to the Kurdish news website Rudaw.
Shirley previously fought against Islamic State in the province of al-Hassakeh in north-eastern Syria, according to Kurdish officials.
In an undated video before his death, Shirely is shown saying: "Let me tell you something. I'm from the United States of America and I say kill 'em all!" in reference to Islamic State fighters.
In May, the US-backed Democratic Forces of Syria, an alliance of Kurdish and Arab insurgents, started an attack to retake Minbij, near the Turkish border, from Islamic State.
Hundreds of foreign fighters are believed to have in the past two years joined Kurdish forces in Syria and neighbouring Iraq to battle Islamic State there.
"For us, these foreign fighters are our blood brothers and they are highly respected among our ranks," Idriss Nassan, a senior Kurdish official, told dpa.
Kurds have set up a monument in the northern Syrian city of Qamishli in honour of foreigners killed in the fight against Islamic State.