Police were called to Modo bar in Concert Square, Liverpool, at about 22:00 BST on Saturday.
The doorman was taken to Royal Liverpool University Hospital, where he is being treated for a serious injury.
Officers believe the offender had approached the bar earlier in the evening with a group, but they were refused entry.
He later returned and assaulted the doorman, police said.
Det Insp Lee Wilkinson, from Merseyside Police, said: "This was a vicious assault on the doorman.
"Medical staff who are treating him have said that he is extremely lucky and this could have been a lot worse."
The offender is described as white, 5ft 11in (1.80m) tall, in his early 20s with a "heavy" build and short brown hair. He was wearing dark tracksuit bottoms, a navy t-shirt and a hoodie.
A doorman has been stabbed in the neck in a "vicious assault" after refusing to allow a group of men to enter a bar.