Secateurs, also known as pruning shears, are a type of scissors for use on plants - and strong enough to prune hard branches of trees and shrubs.
Wiltshire Police said: "We were called to an incident on High Street, Malmesbury, at approximately 3.45pm on April 2.
"It is reported that there has been an altercation before a man has allegedly threatened the victim with a pair of secateurs.
"The man, who is described as in his 70s and was wearing a cap, has walked off with his dog.
"Our enquiries are ongoing but anyone who may have witnessed the incident or have any information that will help us with our investigations is asked to call 101 and quote log 202 of yesterday’s date."