"Machine guns, fighter jets... are not our primary security concern. The single greatest threat to our very existence is climate change," Fiji Defence Minister Inia Seruiratu said.
He was addressing a summit in Singapore which has focused on China-US tensions and the Ukraine war.
Cyclones have repeatedly battered Fiji and other low-lying Pacific countries.
"It threatens our very hopes and dreams of prosperity. Human-induced, devastating climate change," Mr Seruiratu told the forum, called the Shangri-La Dialogue.
Floods in Fiji caused by tropic cyclones have displaced thousands of people in recent years and wrought economic havoc.
Pacific states have urged advanced industrialised nations to do more to combat climate change.
"Waves are crashing at our doorsteps, winds are battering our homes, we are being assaulted by this enemy from many angles," Mr Seruiratu told delegates.
The world is now about 1.2C warmer than it was in the 19th Century - the result of humans burning fossil fuels, which release greenhouse gases, mostly carbon dioxide (CO2).