A masked man and two accomplices robbed two teenagers at knife point near Busselton Jetty on Tuesday night.
The teenagers had finished fishing and were returning to their car when approached by two males and a female at 10.20pm.
The men attempted to enter their car, when the masked man prevented the victim from closing his car door.
Busselton detectives allege the two 19-year-olds were threatened with a knife and demanded to hand over their belongings.
The three offenders, described by detectives as dark skinned, took some property from the vehicle before leaving the area.
Anyone with information to assist in the investigation is asked to call Crime Stoppers on 1800 333 000.