The YPG said Levi Jonathan Shirley, a volunteer also known as Agir Servan, died on July 14 in Manbij, an ISIS-held city in northern Syria.
Shirley first traveled to Syria in February 2015, joining the YPG in Rojava. After a short trip back to the United States, he returned to Syria in January 2016, fighting in Al-Jazira and Kobani before the offensive to recapture Manbij.
The YPG, a Kurdish group of some 30,000 fighters, is the senior partner in the Syrian Democratic Forces, which also includes some smaller Arab and Christian groups.
His mother, Susan Shirley, said her son always wanted to join the U.S. Marine Corps but his eyesight was not good enough.
"He was born with very poor eyesight," she said Thursday. "He had surgery as a teen. Even so, it was not enough to join the Marines."
The last contact Shirley had with his family was in April with his sister. "We were all in the dark," Susan Shirley said.
She added that her son would have wanted "to get the story out about the Kurds."
The family was notified of Shirley's death by the U.S. Consulate in Turkey, his mother said. They called five days after her son's death because "it took a while to verify that it was him," Susan Shirley said.
The United States has not publicly confirmed Shirley's death.
U.S. fighters in Syria
Dozens of Americans and other Westerners are fighting in Syria
, mainly with Kurdish groups.
"It's better to stand up and do something if you think you can help than to just sit back and watch because, 'Hey it's on the other side of the world and not my problem'," fighter Randy Roberts told CNN last year
Last month, American Keith Broomfield was killed fighting alongside Kurdish troops
in Kobani, according to the State Department.