Josh Hanson, of Kingsbury, north-west London, was stabbed in the neck at the RE bar in Eastcote, Hillingdon.
He died at the scene shortly after 01:10 BST on Sunday.
Scotland Yard said about 40 people were in the bar at the time of the attack and urged anyone there after midnight to contact them with information.
No arrests have yet been made.
Det Ch Insp Noel McHugh said: "It is believed there would have been around 40 people present at the time of the incident, many of whom left the venue as paramedics worked to save Josh's life. It is imperative these people contact us to let us know what they saw.
"We are also keen to hear from anyone who was in Field End Road area at around 01:00 Sunday who may have seen or heard anything that they now think might be because of the incident at the RE bar."
A murder investigation has begun over a man who was fatally stabbed in front of dozens of customers at an over-21s bar in north-west London.