Officers were called to the Lobster, on Mossway in Croxteth, at 11.15pm on Friday to reports of a shooting.
The victim was taken to hospital by ambulance, and police now say his condition is stable.
No one else was injured during the incident, which it is believed was targeted although the motive remains unclear.
Detective Superintendent Paul Richardson said: “If anyone has information, but is worried about coming forward I'd like to reassure them that there are special measures that can be put in place to protect witnesses in serious cases such as this.”
