Jordan Chadwick, 31, from Burnley in Lancashire, served with the International Legion in Ukraine since October 2022.
He previously served as a Scots guard in the British Army from 2011 to 2015.
His mother Brenda Chadwick told the BBC her family are “devastated” and “no words can be found to describe the loss of such a short life”.
Mrs Chadwick was reportedly told of her son’s death by Lancashire Police on June 26.
He was repatriated on August 7, as organised by the Ukrainian International Army, the BBC reports.
An inquest into the cause of his death will be held in February.
Mrs Chadwick told the BBC that her son, known as Joe, wanted to be a soldier from a young age.
“His passion to support freedom and assist others with his skills led him to leave the UK,” she said.
She said: “Although we are extremely proud of his unwavering courage and resilience, his death has been devastating.
“No words can be found to describe the loss of such a short life.
“A son, brother, grandson, nephew and uncle, who was loved immensely.”
A Foreign, Commonwealth and Development Office spokesperson said: “We are providing assistance to the family of a British man who died in Ukraine and are in contact with the local authorities.”