The incident took place in the car park of Marston Green Recreation Ground, Bickenhill Road in Solihull, on Wednesday.
Officers were sent to the scene after reports a group of teenagers were attacking each other with machetes at around 4.50pm.
A 17-year-old boy was found with a knife wound to his chest. He has undergone surgery and remains in hospital in a stable condition.
Two other teenagers, aged 17 and 18, suffered minor injuries and have since been discharged from hospital.
A 15-year-old was arrested on suspicion of attempted murder and has been released on police bail as the investigation continues.
Anyone with information is asked to message the force on Live Chat through our website or call the independent charity Crimestoppers anonymously on 0800 555111 and quote log 2631 from August 3.