The 16-year-old victim was walking in Bishopbriggs Park, East Dunbartonshire at around 7.30pm yesterday when he was approached by the two men, police said.
One of them threatened the boy with a knife and demanded his phone. He then stabbed him in the chest and the pair ran off with the mobile.
The boy was taken to Glasgow Royal Infirmary for treatment and was later released, Strathclyde Police said.
Both robbers are white and aged around 19-20.
One is 5ft 9in and slim. He was wearing a grey hooded top and black woollen hat.
The other robber was wearing a dark-coloured tracksuit and had a dog on a lead.
The pair ran off in the direction of Kirkintilloch Road and initial police inquiries have found that they may have been in Tofthill Avenue and Stirling Drive, Bishopbriggs after the robbery.
A force spokesman said officers are carrying out door-to-door inquiries in these areas.
Detective Constable Steven Gault said: "This park is popular with the local community using the amenities, with people out running and people walking their dogs. I am appealing to anyone who may have been in the park around the time of the incident to get in touch.
"It's possible you may have noticed the suspects either prior to or after the incident. Perhaps you saw them run off? If you have any information or knowledge, please do get in touch with us as your information could prove significant to the investigation and help us trace those responsible."
Anyone with information can call police on 0141 532 4400 or anonymously via Crimestoppers on 0800 555 111.