The 16-year-old is understood to have sustained the injuries during a fight between two gangs in Priory Queensway at around 3.30pm. His injuries are not thought to be life-threatening.
One of the attackers is believed to have been holding a machete, West Midlands Police said.
A spokesman for the force said the incident - which was close to Colmore Plaza - told Birmingham Live it was "isolated" and "there was no threat to the wider public".
Police officers remained at the scene as urgent enquiries continue.
They said: "We’re investigating after a teenager was stabbed in Birmingham City Centre this afternoon. We were called to Priory Queensway just after 3.30pm to reports of violence involving a number of people.
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"It’s believed one was carrying a machete, and a 16-year-old boy was taken to hospital with knife wounds which are not believed to be life-threatening.
Officers have already reviewed CCTV and are urgently trying to identify and arrest the suspects.
"At this stage, it's believed to be an isolated incident involving people who were known to each other with no wider threat to the public.
"We remain at the scene tonight while the investigation continues. Contact us via Live Chat at West Midlands Police, quoting log 2623."
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Photos from the cordoned-off streets show belongings left on the ground, including a yellow bag.
An earlier statement from West Midlands Police said: "We’re investigating after a teenager was stabbed in Birmingham City Centre this afternoon.
"We were called to Priory Queensway just after 3.30pm to reports of violence involving a number of people."
A witness at the scene told Birmingham Live that Dale End was also cordoned off by police as people were told to leave the scene.
They added that they could see 10 police cars in the area.
National Express West Midlands buses in the area are being diverted.
They have already reviewed CCTV and are urgently trying to identify and arrest the suspects.
The force said on Twitter: "At this stage, it's believed to be an isolated incident involving people who were known to each other with no wider threat to the public."