Karen Brough, 40, stole more than 8,000 from Dorothy Brough's pension, using part of it to buy carpets and pay a speeding fine.
Brough forged her grandmother's signature on cheques before paying them into her account. As a result, Mrs Brough was so hard up, staff at her warden-controlled home in Market Weighton, East Yorkshire, bought milk and bread for her.
The theft only came to light after the pensioner's death last February.
Brough, of Driffield, pleaded guilty at Hull Crown Court to six counts of fraudulently obtaining money and was sentenced to six months in prison.
Over a three-year period, Brough paid cheques of between 100 and 300 into her own bank account.
Judge Roger Thorn QC told Brough: "How you came to involve yourself with forgery and theft of your grandmother's property absolutely leaves me bereft of understanding.
"You spent the money on carpets and a speeding ticket and it came to light because a carer noticed how little money she had.
"How you could do this to a 97-year-old person beggars belief but to your own family member, your grandmother, is a mystery."
Police launched an investigation after Mrs Brough's carer told them she had noticed how little money she sometimes had.
The stolen money has been refunded to Mrs Brough's estate by her bank.
The court heard Karen Brough was unemployed and had debts totalling 15,000.