Dean Carl Evans of Reading died fighting with the People's Defense Units (YPG) on July 21, YPG spokesman Garib Haso told CNN on Tuesday.
Evans' father, Peter Evans, confirmed the death on Facebook, saying his son had "sadly lost his life ... fighting for our country."
He posted a picture of his son at a younger age in fatigues, saying he "was loved and will be missed by all his family and friends."
"I would like to say a massive big thank you to all my friends and family who sent [their] condolence for the loss of my son," he wrote, adding that Dean "would have been very proud and would have regarded you all as his brothers and sisters."
A Slovenian national enlisted with the YPG, Martin Gruden, was also killed in Manbij on July 27, the YPG announced in a statement.
The Manbij offensive
The YPG is fighting in an alliance with other groups backed by U.S.-led air strikes to liberate Manbij from ISIS control.
Defeating ISIS in Manbij would essentially block a supply route ISIS has between its heartland of Raqqa and Turkey.
The UK Foreign Office said it could not confirm details of Evans' death and again advised British nationals not to travel to Syria.
"As all UK consular services there are suspended, it is extremely difficult to confirm the status and whereabouts of British Nationals in Syria. Anyone who does travel to these areas, for whatever reason, is putting themselves in considerable danger," the Foreign Office said in a statement.
Evans is the second British national to die fighting ISIS in Syria alongside a Kurdish militia.
In March of 2015, Konstandinos Erik Scurfield was killed in Syria after fighting with the YPG for three months.
He was previously an actor on the British drama "Hollyoaks."
An American civilian fighting with the YPG, Levi Jonathan Shirley, died on July 14 in Manbij, the YPG reported last month.