Kathleen Kane sobbed as she said: "I wouldn't lie for him. He wouldn't touch my mother. He adored her."
The evidence was heard at the trial of Garry Kane at the High Court in Dumbarton.
The 41-year-old is charged with murdering Kathleen Millward, 87, at her home in Stonehouse, Lanarkshire on January 3.
Miss Kane was asked by defence QC Gordon Jackson: "You can't think of any reason why he would have done any- thing remotely like he's been accused?"
She replied: "No. I believe him – my heart tells me he didn't do this."
The court had already heard Mrs Millward's body was found in the kitchen of her home.
Miss Kane said she got a phone call from her sister telling her that their mum was dead and she drove over to her mother's house.
She said her sister and brother-in-law were standing outside the house and Garry was in the bedroom.
She added: "He was very, very upset. He just kept saying 'my gran, my gran'."
The murder charge includes claims that Garry Kane struck the OAP on the head with a blunt implement and applied pressure to her neck. Kane is also alleged to have stolen money from the property.
He is further accused of possessing heroin and three theft charges. Kane denies all charges. The trial continues.