A teenager was threatened with a knife and mugged in the Oakes area of Huddersfield on Tuesday May 8.
The robbery happened near a bus stop on Tanyard Road, close to Oakes Road South, between 4.10pm and 4.25pm.
The victim was on foot when he was approached by three males. One of the males threatened the 16-year-old with a knife and then they stole his phone and bag.
A 17-year-old male has since been arrested on suspicion of robbery and released under investigation.
West Yorkshire Police are appealing for witnesses to come forward and particularly want to speak to a woman who pulled up alongside the robbery and told the three robbers to stop what they were doing.
The woman, or anyone else who has any information, is asked to call PC Amy Lane of Kirklees CID on 101 quoting log 1379 of 8 May.
Information can also be given anonymously to the independent Crimestoppers charity on 0800 555 111.