The incident happened in Keptie Road, Arbroath, between 6pm and 6:30pm on Monday (September 20).
The man, believed to be in his late 40s, allegedly pulled the teen off his bike and threatened him after he cycled past him at Keptie Pond.
A scuffle then ensued, and a member of the public intervened before the man walked off towards Timmergreens.
Police Scotland said in a statement: “We would like to trace a man described as between 45-50 years of age, about 5ft 9, stocky build with short grey hair, who was wearing a black jacket and black bottoms, and a beanie-type hat.
“We would also like to trace the member of the public who intervened and assisted the complainer.
“If you have any information that could assist our investigation, please call 101 or speak to any police officer.”