The boy was threatened with a knife by one of two men who demanded he hand over his mobile phone during the robbery on Thornhill Avenue in the Lindley area of Huddersfield at around 11pm on August 1.
A West Yorkshire Police spokesman said: "As the suspects were leaving, one of the males slashed the victim with the knife causing him to sustain a two-inch wound."
PC Paul Jackson, of Kirklees CID, said: "This was a distressing attack on the victim and it was lucky that he wasn’t more seriously injured.
“The victim had already handed over his belongings, but the suspects still proceeded to slash him with a knife."
The suspect with the knife is described as mixed race, in his late teens, around 6ft tall, of medium build and was wearing a black puffer jacket.
The second suspect is described as mixed race, in his late teens, around 5ft 3in tall and of stocky build. He had light brown curly hair, which was short at the sides and longer on top. He was wearing a grey zip-up hooded cotton top.
Anyone who has any information is asked to contact Kirklees District CID via 101, quoting crime reference 13170353666.