Cyclone Josie caused severe flooding when it passed Fiji's main island Viti Levu, with Bainimarama confirming Tuesday that the death toll from the storm had risen to five.
"Once again, Fijians are mourning the loss of some of our loved ones to an extreme weather event," he told a function in the capital Suva.
Bainimarama said climate change meant powerful cyclones were battering Pacific island nations with increasing frequency.
"We are now at an almost constant level of threat from these extreme weather events that are becoming more frequent and more severe because of climate change," he said.
He added: "As a nation we are starting to build our resilience in response to the frightening new era that is upon us."
Fiji took a leading role in global climate talks last November when Bainimarama became president of the UN's COP23 negotiation in Bonn.
The Fijian leader said Cyclone Josie underscored the urgency of dealing with climate change.
"We need to get the message out loud and clear to the entire world about the absolute need to confront this crisis head on," he said, describing climate change as "a fight for our very survival".
Fiji was hit by Cyclone Winston in 2016, which killed 44 people and wiped out one-third of the country's economic production.
It escaped the worst of Cyclone Gita in February but areas of neighbouring Tonga and Samoa were devastated.
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