The U.N. chief told reporters that "this is even more significant because the previous hottest month, July 2016, occurred during one of the strongest El Nino's ever," which was not the case this year.
An El Nino is a natural warming of the ocean that once it interacts with the atmosphere often warms up the globe and changes rainfall and temperature patterns, making some places wetter and some places drier.
Guterres said the latest weather data, including temperature-shattering records from New Delhi and Anchorage to Paris, Santiago, Adelaide, Australia and the Arctic Circle, means the world is on track for the period from 2015 to 2019 "to be the five hottest years on record."
He warned that if all nations don't take immediate action to tackle climate change, extreme weather events happening now "are just the tip of the iceberg."
"And the iceberg is also rapidly melting," Guterres said.
Arctic Sea ice is already near record low levels, he said, and the European heat wave last month has raised temperatures in the Arctic and Greenland by 10 to 15 degrees Celsius.
"Preventing irreversible climate disruption is the race of our lives and for our lives," Guterres said. "It is a race we can — and must — win."