West Midlands Police said the man in his thirties died after being stabbed last night on a bus in Handsworth.
Police were called at 10.43pm following reports a passengers had been attacked.
The victim was found with stab injuries on the upper deck of the bus.
Police say the man has not been formally identified and died from his injuries a short time after the attack.
Officers are carrying out urgent investigations to trace the offender, described as a black man, aged around 30 years and wearing a baseball cap and padded jacket He is believed to have left the bus in Rookery Road.
Temporary Superintendent Tom Chisholm from Force CID said: “The investigation is in the early stages but it is believed the man was stabbed following an altercation between him and the offender on the upper deck of the outer circle No 11A bus as it travelled along Rookery Road in Handsworth.
“I am appealing for anyone who was travelling on the bus last night and who has information or anyone who saw the offender after leaving the bus to call police."
This morning Rookery Road, together with part of Oxhill Road and Greenhill Road, remains closed while forensic enquiries take place and is likely to remain closed for the rest of today.
Detectives continue to trawl CCTV and carry out local enquiries.
More to follow…