Fire crews have been called to Guildhall Road tonight where they are attending an ongoing incident.
Eyewitness Gareth Eales has reported a "horrific smell of smoke" at the top of Guildhall Road and three fire engines are now in attendance.
Guildhall Road has been taped off to oncoming motorists and traffic delays have been reported on St Giles Street.
Bartender at The Mailcoach Eaymen Jones said: "They have taped off the street and coned off the road, Derngate is still open.
"As soon as you step outside of the pub all you can smell is smoke."
A spokeswoman for Northamptonshire Fire & Rescue Service confirmed the force is attending to a fire at the museum.
She said: "We are attending a fire there. We have four appliances on scene and it is ongoing at the moment."
Cllr Jonathan Nunn, leader of Northampton Borough Council, said: "We're extremely grateful to Northamptonshire Fire and Rescue for their quick response, and to whoever called the incident in to the Fire Control Room.
"We will now have to wait for the Fire Service to provide feedback from their investigation."
Updates to follow.