Devon and Somerset Fire and Rescue Service has said the blaze is at the marina in Torquay.
Five crews are said to be at the scene battling the fire.
The boat is described as a ‘85ft’ vessel.
The fire service tweeted: ‘We currently have 5 fire appliances at #Torquay marina fighting a fire on an 85ft private vessel.
‘Please do keep away from the area. Details will be added to the Fire Service Newsroom.’
ITV News has reported that the boat on fire is a ‘£6m superyacht’.
Plumes of black smoke have been seen billowing from the harbour this afternoon.
Pictures from the scene capture the scale of the blaze, which can be seen in the embedded tweet in this article.
One witness told ITV they had heard ‘explosions’ following the start of the fire.