"The danger is especially acute for nearly 900 million people who live in coastal zones at low elevations -- that's one out of ten people on Earth," Antonio Guterres told the UN Security Council.
"Low-lying communities and entire countries could disappear forever... We would witness a mass exodus of entire populations on a biblical scale," he said.
It is not only small island states at risk as sea levels rise, he added.
Countries such as Bangladesh, China, India and the Netherlands are all in danger, Guterres said, while "mega-cities on every continent will face serious impacts" -from Cairo to Jakarta to Los Angeles to Copenhagen.
The UN's Intergovernmental Panel on Climate Change (IPCC) says sea levels rose by 15-25 centimeters (6-10 inches) between 1900 and 2018.
If the world warms by just two degrees Celsius (3.6 degrees Fahrenheit) compared to the pre-industrial era, then those levels will rise again by 43 centimeters by the year 2100.
But if it warms by three or four degrees Celsius, sea levels could rise by as much as 84 centimeters, the IPCC says.
The problem must be addressed "across legal and human rights frameworks," Guterres warned.
Rising sea levels means shrinking land mass, he said, which could drive possible disputes over land and maritime space.
"The current legal regime must look to the future and address any gaps in existing frameworks," including in international refugee law, he said.
It must also provide for the future of states that face losing their land territory completely.
Guterres said the Security Council has a "critical" role to play in addressing "the devastating security challenges arising from rising seas."
The issue has been controversial in the past: In 2021, Russia vetoed a resolution linking climate change and global security, which was supported by the majority of the Council members.