Zubeidat Tsarnaev replayed a recording of the conversation with her son, who is being held in Fort Devens prison in Massachusetts, in which he says "everything is fine" ahead of his trial.
"I'm already eating and have been for a long time," he said in the message, which was recorded last week according to Channel 4.
"They are giving me rice and chicken now. Everything is fine," he added. "Everything is good."
Their mother insisted to the British news channel during the interview in Dagestan that "I know my kids didn't do it".
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She also explained that Djokhar had told her: "Don't worry about me, I do have money".
He explained that "somebody opened an account for me and people send me money here, I do have lots of money."
She added that he said thousands of dollars when asked him exactly how much.
"There is no justice," he added. "It's impossible to understand what happened."
The parents' campaign protesting their sons' innocence has raised USD 8,000, they revealed.
Tamerlan Tsarnaev died in a gun battle with police three days after the April 15 bombings that killed three people and injured more than 260 at the marathon finish line.
A first court hearing for Djokhar Tsarnaev was last month pushed back to from July 2 to July 10.