The 55-year-old stopped in her tracks to take photographs of the spooky green-faced character which had been attached to the passing place sign.
The roadside display was alongside a massive spider attacking a man, a person nailed to an electricity pole and a pumpkin character killing someone with an axe.
Alisa posted photographs and a video of the display onto Facebook last night, writing: “Happy Halloween.”
The post has gained over 1,000 likes and dozens of comments from humoured social media users.
Jackie Baxter said: “That is fantastic – so realistic.”
June Patterson wrote: Love this. Made me laugh. Well done.”
Chris De Gendt commented: “You rock, only in bonnie Scotland. “
And Andrew Forrester added: “One word, awesome.”
Speaking today, Alisa said: “We heard there were Orca on the west mainland of Shetland and drove like crazy to try and find them and in the process came across this amazing witch.
“I do not know [who made it], but what a fantastic creation with her luminous face and all.
“I had to stop at this passing place and grab a quick video and a couple of pics.
“I shared it on Facebook and didn’t expect such an interest.
“I hope the creator sees how popular their witch is.
“Shetland seems to create amazing Halloween structures all dotted along the roadside for all to see and enjoy.”
Shetland has its own Halloween celebrations, an older form of guising, called Skekling.
The event usually begins around the first Sunday after Winter Day, marking the beginning of Winter on the Island.
Little is known about the origins of the practice, but it is said to be a mixture of Celtic and Norse costume-wearing tradition.