Garrett Foster was pushing his fiancée's wheelchair at the protest when a car drove into the crowd, his mother Sheila Foster told US media.
As demonstrators approached the car, someone inside the vehicle opened fire on the group.
Mr Foster was taken to hospital, but was pronounced dead shortly afterwards. Police have arrested a suspect.
On Saturday, marches were held in a number of cities in solidarity with activists in Portland - Austin; Seattle; Louisville, Kentucky; Aurora, Colorado; New York; Omaha, Nebraska; Oakland and Los Angeles in California; and Richmond in Virginia.
Sheila Foster told ABC's Good Morning America on Sunday that her son and his fiancée, Whitney Mitchell, had been participating in protests against police brutality for more than 50 days.
"He was doing it because he felt really strongly about justice and he was very heavily against police brutality, and he wanted to support his fiancée," she told the programme "His fiancée is African-American."
In a statement shortly after the shooting, a police spokesperson did not identify the victim but said he had been seen carrying a rifle. The New York Times later reported that he had been carrying an AK-47, as had other protesters.
Carrying rifles is common at protests in Texas, which has liberal gun laws.
Ms Foster added that it "wouldn't surprise" her if her son had been carrying a gun, as he had a licence to carry and "he would have felt the need to protect himself".
Before the shooting, Mr Foster spoke to independent journalist Hiram Gilberto Garcia about carrying the rifle at the protest.
"They don't let us march in the streets anymore, so I got to practice some of our rights," he said, on a social media live stream. "(But) if I use it against the cops, I'm dead."
