The victim, aged 14, was attacked in Abbey Park, in the Gornal area of Dudley, on Wednesday night, suffering a cut to his knee.
Police arrested a teenage boy, 15, on suspicion of wounding and released him under investigation.
A spokesman for West Midlands Police said: "A 15-year-old boy has been arrested on suspicion of wounding after another teenager was cut across his knee with a knife at around 6pm Wednesday.
"The 14-year-old boy was not seriously injured and did not require hospital treatment.
"Following enquiries the teen was arrested this morning and has been released under investigation."