North Wales Fire and Rescue Service said the blaze on Victoria Pier in Colwyn Bay started due to hot metal work being carried out by contractors.
About 10 firefighters used cherry pickers to reach the fire, which broke at about 16:30 BST on Friday.
A crew remained on-site overnight as the tide restricted access and they could not be sure the fire was out.
Part of the 116-year-old structure, which was already closed to the public for safety reasons, collapsed into the sea last month.
It was further hit by Storm Doris three weeks later.
Work to demolish the damaged parts of the pier, which has been closed since 2008, began earlier this month.
The remainder of the seaward section is to be redeveloped into a shortened boardwalk with plans to restore its 1930s Art Deco pavilion also being looked at.
Conwy council voted to demolish the structure in 2013 and a report at the time said restoring the pier would cost more than Â£15m.
But demolition was refused by the Welsh Government in 2015.
A fire at a disused Grade II-listed pier in Conwy county was accidental, the fire service has said.