The incident took place at approximately 1-15am on Saturday, November 26th, 2016, at The Turf public house in Yorkshire Street.
A man has been arrested and is currently on police bail.
PC Carla Martin, of Burnley Police, said: "This is a serious assault whereby a male has been attacked resulting in serious facial injuries. I would urge anybody who saw or heard anything to contact us."
Please contact Burnley Police on 101 quoting log 83 of 26/11/16 or calling Crimestoppers, anonymously, on 0800 555 111.