Caroline Grills (c. 1890 - 6 October 1960) was an Australian serial killer. She was nicknamed Aunt Thally due to her kindly demeanour and tendency to kill victims with thallium poisoning.
Grills was born around 1890 in Sydney. She first became a murder suspect in 1947 after four family members - her stepmother, two relatives by marriage and her sister-in-law - died mysteriously. Authorities tested tea Grills had given three additional family members and detected the rat poison thallium. Traces of thallium were also found inside the bodies of the victims.
Grills was charged with four counts of murder and three counts of attempted murder. She was convicted on 15 October 1953 and sentenced to death, but her sentence was commuted to life imprisonment soon after and she was incarcerated in Long Bay prison, where she was affectionately nicknamed "Aunt Thally" by the other prisoners.
On 6 October 1960 Grills suffered a ruptured gastric ulcer and was rushed to the Prince Henry Hospital, where she died of peritonitis.