The attack happened at Allanmore Farm, Drumnadrochit, at about 07:30 on Tuesday.
At least three sheep have since died as a result of their injuries, officers said.
The white and brown spaniel is believed to be called "Max". Anyone who recognises the dog is asked to contact the police.
Sgt Craig Stuart said: "Police Scotland are keen to trace the owner of the dog to assist with the inquiry. The dog is safe and well and being looked after locally."
Police have caught a dog they believe killed three sheep on a Highland farm and are now trying to trace its owner.