The victim, in his 40s, remains in hospital following disorder outside the Phoenix pub, in Martley Road, Oldbury, at 9.10pm on Saturday.
West Midlands Police has asked people involved in the fight and witnesses to come forward.
A spokesman for West Midlands Police said: "It’s understood a man, aged in his 40s, was punched in the face and suffered a serious head injury as he fell to the floor.
"He’s been taken to hospital where he remains in a serious condition.
"Our officers conducted initial enquiries at the venue last night and have seized CCTV for examination.
"We’re appealing for witnesses who’ve not yet spoken to us to come forward – and we would also urge the people involved in the fight to come and speak to us so we can hear their side of the story."
Anyone with information is asked to contact police quoting crime reference number SW/260613P/20.