Firefighters were called to gastropub The Lighterman in Granary Square shortly after 5.30pm on Wednesday.
Photos posted on social media showed a mass of white smoke rising into the air around the building.
Half of the ventilation system in the pub was alight, according to the London Fire Brigade.
A spokeswoman said: "Four fire engines and 21 firefighters and officers have been called to a fire in a pub in Granary Square.
"Currently 50 per cent of the restaurant ducting – the ventilation system – is alight."
A spokeswoman for KCCLP says: “Customers of The Lighterman were promptly evacuated and we are working with the tenant to establish the cause of the fire.”
Granary Square is home to a number of pubs, restaurants and cafés and is a popular after-work destination.