The 18-year-old was left with back and face injuries after being attacked in Booth Street, Handsworth, just before 2pm on Wednesday.
The knifemen are believed to have fled in a car towards Soho Road, West Midlands Police said, after the attack happened near Crocketts Road and the Booth Street tram stop.
The area was cordoned off as forensic specialists examined the scene and so far no arrests have been made.
Meanwhile the victim was said to be in a stable condition in hospital on Wednesday evening.
A West Midlands Police spokesman said: "
This was a shocking attack in broad daylight and we are asking for anyone with information who hasn’t already spoken to us to get in touch quoting log 2111 of 7/4.
West Midlands Ambulance Service sent an ambulance, a paramedic officer and a critical care paramedic and doctor to treat the man.
A spokesman for the service said: “Crews treated one patient, a man, for injuries not believed to be life threatening and he was taken to hospital on blue lights and sirens for further treatment.”