Emergency services rushed to Henry Barber Park and Imperial Road that runs alongside it, in the Bordesley Green area, on Saturday evening.
As well as the 15-year-old boy, three other teenagers, two aged 19 and one 18, were also stabbed.
All four were taken to hospital where they are said to be in a stable condition.
West Midlands Police carried out searches of the scene in the evening and detectives joined officers to investigate the incident.
A spokeswoman for West Midlands Police said: “We were called to reports of disorder and fighting in Henry Barber Park and nearby Imperial Road at around 6pm yesterday evening.
“Two men, both aged 19, as well as an 18-year-old man and a 15-year-old boy were taken to hospital with stab wounds.
“None of their injuries are thought to be life-threatening or life-changing at this time.
“All four have been spoken to by officers, we’ve also recovered forensic evidence from the scene.
“Our enquiries are currently ongoing to establish exactly what happened and who was involved.
“We understand that a large number of people were in the area at the time and anyone who witnessed what happened, or has any other information should get in touch.
“Please contact us via Live Chat at www.west-midlands.police.uk 8am to midnight or call 101 anytime. Please quote crime number 20BE/253580R/20.”
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org