Four acres of marshland were believed to be deliberately set alight at Carlton Marshes Nature Reserve.
A total of six different fires started at the Suffolk Wildlife Trust lowland reserve at Carlton Colville, near Lowestoft, at around 2.45pm.
Four fire crews tackled the fires which took more than two hours to get under control.
Equipment, including fire beaters, were used to contain the fire.
A fire service spokeswoman said police had been informed because the blazes were thought to be suspicious.
Three engines from Lowestoft South and one from North Lowestoft attended.
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