West Midlands Ambulance Service said the crash, between a single and a double decker, happened on Addison Road, Kings Heath just after 11:00 BST.
Both drivers had to be cut free from their cabs, while four passengers were taken to hospital.
National Express, who operate the buses, has apologised to passengers and vowed to find the cause of the crash.
The two buses - a number 11 and a number 27 - are believed to have been following their usual route when the crash happened.
West Midlands Ambulance Service said the drivers had escaped "serious injury".
The service said: "Crews arrived on scene to discover two buses had been in collision, with the front driver's side of both vehicles having suffered significant damage.
"It quickly became evident both drivers were trapped."
They said both men, believed to be in their 40s, were treated for leg injuries before being taken to Queen Elizabeth Hospital in the city.
"Both men appear to have escaped serious injuries which, given the significance of the collision, is extremely fortunate," the service said.
"One of the passengers on the single decker bus, an elderly female, had to be helped from the vehicle on a stretcher. She was treated for back injuries and taken to the same hospital.
"A man from the single decker bus was treated for elbow, shoulder and knee injuries, whilst two females were treated for minor injuries."
A further seven passengers were assessed and discharged at the scene.
National Express said: "We can confirm two of our vehicles were in involved in an incident this afternoon.
"We are sorry to those involved and will monitor the situation to ensure they have what they need. We are working with the authorities to assess the cause of the incident.
"We also apologise to local residents who are suffering from diversions and congestion."
Both buses are likely to have been fitted with CCTV.
The company said it was in the process of retrieving the buses.
Six people have been injured in a head-on crash between two buses in a Birmingham street.