Kathleen Kane said her son Garry Kane plans to appeal against his conviction and should never have been found guilty for the murder of her mother, Kathleen Milward, 87.
Kane, 41, must serve at least 17 years before he is eligible to apply for parole after beating the pensioner to death in her bungalow in Stonehouse, Lanarkshire, on January 3 last year.
The High Court in Glasgow was told drug addict Kane attacked Mrs Milward in a desperate bid to get money for his next fix.
However, Ms Kane rejected this, claiming that her son was a loving and a caring man who doted on his grandmother.
Speaking after the court hearing yesterday, she said: "In my opinion Garry is innocent of the murder of his gran.
"This opinion is also held by Garry's friends and my friends who know Garry, who knew his close relationship with his gran and knew how he felt about her.
"Garry is a kind and caring person and adored his gran and the stories that have been put out there about the opposite being true are completely false."
Ms Milward also paid tribute to her mother, whom she described as her "friend and confidante".
She said: "Probably if I had read in the press of a mother standing behind her son who was accused of murdering her mother, I would have thought that she was doing this only because it was her son.
"But Garry knows that would not happen with me.
"I would not defend my son if I thought for one minute that he was guilty – but I know he is not."
However, Kane's aunt, Maureen Kennedy, said she believed that he was indeed the killer.
She said: "The sentence will change nothing for me. I am just sad for the whole family. Nothing will bring my mother back."
Sentencing Kane, Judge Lord Matthews told him: "You were found guilty of the murder of your own grandmother. No words of mine can bring home the enormity of that."