The bearded figurine, measuring 52inches in height, is wearing a helmet, armour and draped sash and is holding an oval shield at his side.
The historic character, which has some parts missing, will go under the hammer at Railton’s Northern Auction Centre, in Wooler, on Saturday.
The piece was consigned from a property across the Border and is expected to make in the region of £400 to £600.
Auctioneer Jim Railton said: “The bearded stone figure is thought to be 18th century, but there was much remodelling of these iconic figures staring down from the battlements of the castle.
“As a result, several have ‘walked’ to local houses and gardens around Alnwick over the years and we have sold them several times before. This bearded soldier seems to be wearing a skirt, so we assume he is a Scottish variant.”
The soldier will be auctioned as part of the May antiques sale at the Centre, on South Road, which runs from 10am. To view the catalogue, which features hundreds of lots, visit jimrailton.com or call 01668 283000 for more information.