The Great Amherst Mystery was a notorious case of reported poltergeist activity in Amherst, Nova Scotia, Canada between 1878 and 1879. It was the subject of an investigation by Walter Hubbell, an actor with an interest in psychic phenomena, who kept what he claimed was a diary of events in the house, later expanded into a popular book.

The Amherst Mystery centred on Esther Cox, who lived in a house with her married sister Olive Teed, Olive's husband Daniel and their two young children. A brother and sister of Esther and Olive also lived in the house, as did Daniel's brother John Teed.

According to Hubbell's account, events began at the end of August 1878, after Esther Cox, then aged 18, was subjected to an attempted sexual assault by a male friend. This left her in great distress, and shortly after this the physical phenomena began. There were knockings, bangings and rustlings in the night, and Esther herself began to suffer seizures in which her body visibly swelled and she was feverish and chilled by turns. Then objects in the house took flight.

The frightened family called in a doctor. During his visit, bedclothes moved, scratching noises were heard and the words "Esther Cox, you are mine to kill" appeared on the wall by the head of Esther's bed. The following day the doctor administered sedatives to Esther to calm her and help her sleep, whereupon more noises and flying objects manifested themselves. Attempts to communicate with the "spirit" resulted in tapped responses to questions.

The phenomena continued for some months, and became well known locally. Visitors to the cottage, including clergymen, heard banging and knocking and witnessed moving objects, often when Esther herself was under close observation. In December Esther fell ill with diphtheria. No phenomena were observed during the two weeks she spent in bed, nor during the time she spent recuperating afterwards at the home of a married sister in Sackville, New Brunswick. However, when she returned to Amherst the mysterious events began again, this time involving the outbreak of fires in various places in the house. Esther herself now claimed to see the "ghost", which threatened to burn the house down unless she left.