Garry Kane, 41, a former soldier, denies killing his grandmother Kathleen Millward, 87, at her home in Stonehouse, South Lanarkshire on January 3.
Mrs Millward's daughter Kathleen Kane told the High Court in Dumbarton: "I believe him. My heart tells me he didn't do this."
Ms Kane was the first witness in the trial of her son.
She told the court her mother lived in a semi-detached bungalow with her West Highland terriers after the death of Ms Kane's stepfather, Robert.
The jury heard Garry Kane moved into the house in the final years of Mrs Millward's life.
Kane, who was a heroin addict, had left his family home as a teenager. He served in the Army and lived in England before returning to Scotland in 2007.
At first he lived with his mother but the court was told she threw him out after he stole electrical goods and jewellery from her.
Ms Kane broke down and sobbed as she told the court her son did not kill his grandmother.
She said "I wouldn't lie for him or cover up for him because he's my son. He wouldn't touch my mother, he adored her. Apart from myself he was the only affection she got.
"He's always been there for her. I'm not saying he's perfect, he has his problems."
She was asked by defence QC Gordon Jackson: "You can't think of any reason why he would have done anything remotely like he's been accused?"
She replied: "No, if I thought he had done that I would not cover up for him."
Advocate depute Jennifer Bain asked Ms Kane if she had any worries about Kane stealing from his grandmother while he was living there.
She replied: "He definitely wouldn't. He had immense respect and love for my mother. He would never have stolen from her."
Ms Kane said the last time she spoke to her mother – who was frail and suffering from an increased heart rate – was on the phone on the afternoon of the day she died.
Asked why she phoned her mother, she replied: "I was in a way checking up on her. It was something I did all the time."
The witness said she could not remember the full details of the phone call.
Ms Kane told the court that at around 7pm she tried to phone her mother again and got no answer.
She said: "I was a bit anxious but because it was new year I wondered if she had gone to the neighbours."
After trying three times she phoned her son, who she believed was at a friend's house, and asked him to check on his grandmother.
He agreed to do so and eventually phoned his mother back, the jury was told.
Ms Kane said: "He was extremely upset. I could hardly make out what he was saying. He said there was something wrong with my mum."
The court also heard that the accused had fallen out with a family member. He had been accused of taking money from Christmas cards meant for Mrs Millward's children.
It is alleged Mr Kane struck the pensioner on the head with a blunt implement and applied pressure to her neck, and stole money from the house.
He faces separate charges of behaving in a threatening manner towards his grandmother, possession of heroin and theft.
Mr Kane denies all the charges.
The trial before judge Lord Matthews continues.
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