Crews were called to a building site in Bloomery Circle, Llanwern, at 21:15 BST on Wednesday.
The blaze burned for more than 10 hours before it was extinguished.
It is understood the burning material included a large amount of scrapped railway sleepers.
A Gwent Police spokesman said: "Officers are investigating the circumstances surrounding the fire... which is believed to have been deliberately set."
A fire involving about 100 tonnes of waste wood and rubbish in Newport was started deliberately, the police and fire service have said.