The U.S. agency NOAA calculated the worldwide average temperature for the month at 60.2 degrees Fahrenheit, which was 1.2 degrees above normal.
That tied 2005 for the hottest September and also made it the 331st consecutive month that global temperatures were above the 20th-century average.
One exception was the United Kingdom, which experienced its coolest September since 1994.
Alaska had its fifth-wettest September since records began in 1918, according to NOAA.
Australia recorded its warmest September days on record, with the monthly high temperature average reaching 3.5 degrees Fahrenheit above normal.
The month’s warmth coincided with Arctic sea ice melting to its greatest extent on record, while its Antarctic counterpart grew to a record expanse.
The latter was in large part due to the annual peak of the ozone hole above the frozen continent and increased polar surface winds collecting the sea ice.