The man was stabbed at around 1am on Sunday on the car park of the The Perry Hill Tavern on Perry Hill Road in Oldbury.
He went on to seek refuge inside the pub where staff called an ambulance for him, though the venue says the incident has nothing to do with them.
A second man involved with the incident also suffered injuries, but neither are thought to be life-threatening.
All five men have been arrested on suspicion of wounding and remain in custody.
A spokesperson for West Midlands Police said: "We were called by paramedics to Perry Hill Road in Oldbury at around 1.10 am after a man was stabbed on a pub car park.
"The 21-year-old man suffered injuries which are not believed to be life-threatening.
"He sought refuge in the pub where staff called for an ambulance. A second man suffered less serious injuries in the disturbance on the car park.
"Five other men have been arrested on suspicion of wounding and remain in custody today."
Police have asked anyone with information or mobile phone footage to get in touch via Live Chat at the force's website quoting log 201 of July 3, or to contact Crimestoppers anonymously on 0800 555 111."