Colwyn Bay's Victoria Pier was already closed to the public for safety reason and there is no suggestion anyone was nearby at the time.
In 2013, Conwy council voted to demolish it but this was refused by the Welsh Government in 2015.
The council said it was responsible for the clearance and a clear-up would start at low tide on Thursday.
A spokesman said: "Contractors have been mobilised to make the area safe, to remove the fallen pieces and salvage materials wherever possible.
"Over recent years the council has been quite clear about the deteriorating state of the pier. Fencing has been in place around the structure for some time for public safety and we urge the public to take note of the situation and keep away from the area."
The pier has been the subject of a protracted legal battle between the council and businessman Steven Hunt.
Mr Hunt failed in a 2015 High Court bid to regain ownership of it when Mr Justice Morgan said the pier was unsafe, in danger of collapsing and Mr Hunt was in no financial position to restore it.
It is estimated it would cost about Â£12m to restore and the council could be asked to provide Â£850,000 towards it.
Campaign group Colwyn Victoria Pier Trust wants the authority to save the structure, saying it would be far less costly than demolishing it.
A Grade II-listed pier has partially collapsed into the sea off the Conwy coast