Firefighters said a patch of gorse had been set on fire deliberately.
One appliance from Perranporth and another from St Just and St Ives were called to Madron, outside Penzance, at 10pm last night (August 15) after a large area of gorse caught fire.
A spokesperson for Cornwall Fire and Rescue Service said the area that was well alight was about 25sq metres in size, which is about five metres by five metres.
The crews used hosereel jets to bring the blaze under control and used handsaws to cut firebreaks.
The spokesperson said: "The fire was believed to be arson and the police have been informed."
It was not the only incident firefighters in Cornwall had to deal with yesterday.
At about 6.54pm, crews from Truro and Tolvaddon (Camborne) were called to a grass and gorse fire measuring about 200m by 400m in Tregavethan, a hamlet on the outskirts of Truro.
Earlier in the day, at around 3.12pm, four crews were called to Mousehole near Penzance to deal with an ongoing fire in a wooded area after a bonfire spread along the highly peaty forest floor.
The original fire happened on Friday but, despite being spotted early by a walker and firefighters tackling it repeatedly, it has reignited and spread again.
In a post on Facebook, the Penzance Community Fire Station said: "It shows the nature of these sort of fires and how difficult they are to extinguish.
"One bit of smouldering soil and a change of wind direction and speed and the end result is three days later we have five acres of wildfire in an extremely difficult spot to access and multiple fire crew and pumps on scene."
These incidents come as a national alert was sent to mobile devices on Friday night warning of the increased fire risk associated with bonfires, disposable BBQs and the careless disposal of cigarettes.
From 7pm on Friday (August 14), the warning was sent to UK residents, along with a loud alert tone on several occasions.
The message warned against lighting anything that could spark a wildfire, including barbecues, campfires, and fireworks.
Recipients were also urged to report fires immediately to 999.
It came after Prime Minister Andy Burnham called in the military, announced a ban on disposable barbecues and warned smokers not to risk setting light to “tinderbox” Britain as he scrambled to respond to the summer heatwave and wildfires.
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Penzance Gorse Fire Treated as Arson After Deliberate Blaze