The message was passed to the Vocativ website by a friend. It was also posted to the “Help for Dzhokhar” site hosted on a Russian social networking site, VKontakte.
In it she thanked supporters.
“Tell everyone that I will never forget their support," she said. "May the creator bless them all for their support of my precious boy."
Her son was found guilty by a Boston jury on Wednesday of all 30 counts he faced - 17 of them carrying the death penalty.
The same jurors will now be asked to decide whether the 21-year-old should be executed by lethal injection.
Tsarnaev, a US citizen who came to America as a child refugee from Kyrgyzstan, stood with his slightly bowed as he listened to the verdict being read.
He was 19 at the time of the bombing and the trial heard how he planted two homemade explosives with his elder brother Tamerlan at the race finish line on April 15, 2013.
The pair fled the scene of the bombing and three days later murdered a university police officer in a failed attempt to steal his gun. They were finally cornered on a suburban street where they tried to fight off police with a pistol and explosives.
His mother, a Russian native who left the US for Dagestan in 2012 after being charged with shoplifting, has always insisted that Dzhokhar and his late brother Tamerlan were innocent.
In a 2013 interview with The Telegraph, she said her son had been set up.
"It`s a big show, a spectacle. Americans love a show," she said.
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