Around 4 p.m., the three juveniles were walking together on the Iron Horse Trail near Civic Drive and Pine Street when the suspect approached them, according to police.
He demanded property from one of the juveniles while brandishing what appeared to be a switchblade, police said. The victim handed over his belongings and the suspect fled on foot westbound on Parkside Drive toward Jones Road.
Officers searched the area for the suspect but came back empty handed. There were no injuries during the incident.
Police describe the suspect only as a black male between 17 and 18 years old, 5 feet 9 inches tall and about 130 pounds. He was last seen wearing a white T-shirt and black basketball-type shorts, police said.
Anyone with information about the incident should call Walnut Creek police at (925) 943-5844.