The incident happened at around 1.10pm on Friday, April 9, as the 15-year-old boy was riding his bike on the track near to the Union Canal, south of Wynford Brae in Philipstoun.
As he cycled, a man approached the teenager from behind some wooden logs and threatened him with a knife. The teenager dropped his bike and ran off towards the canal on foot. The suspect then got on the bike and cycled away along the path towards Linlithgow.
The suspect is described as a white man, 5ft 11ins to 6ft tall, of slim to medium build and had a balaclava covering his face. He was wearing black Ray Ban sunglasses, a two-tone hooded jacket, Nike slim fit tracksuit bottoms and black Nike Airmax trainers.
The stolen bike is a green Saracen Aries 60 Elite mountain bike with distinctive gold coloured suspension (pictured).
Detective Constable David McDougall, of Livingston CID, said: “The teenager has reported that he warned two women about the suspect as he was running from the scene. We are especially keen to trace and speak to these two women as they may have seen the suspect or have information to help our investigation.
“I’d also ask anyone who recognises the description of the suspect, or knows of someone who has recently acquired a green Saracen Aries 60 Elite, to get in touch with police to help our enquiries. Anyone with any information should contact officers as soon as possible.”
Those with information should contact Police Scotland on 101 and quote incident number 1617 of 9 April 2021. An anonymous report can be given to Crimestoppers on 0800 555 111.