Last month, the average combined land and ocean surface temperatures were 0.67 C higher than the 20th century average of 15.6 C, the National Oceanic and Atmospheric Administration said Monday.
That temperature average matched the one from September 2005, which was the warmest September on record since 1880, it added.
Higher-than-average monthly temperatures were most notable in western Canada, central Russia, Japan, western Australia, northern Argentina, Paraguay and southern Greenland, the NOAA said.
Year-to-date global land and ocean surface temperatures were the eighth-warmest January to September period, with record warmth "observed across the eastern two-thirds of the United States and south-central Canada," the NOAA said in its global analysis report.
Last month also marked the 36th consecutive September and the 331st consecutive month that the average global temperature was higher than the 20th-century average.
Some scientists point to human-caused global warming and the loss of Arctic sea ice as the possible reason.
"What's playing out is precisely what climate said we should expect to see 20 to 30 years ago," University of Victoria climate scientist Andrew Weaver told The Associated Press.
Weaver and NOAA climate monitoring chief Deke Arndt said two weather factors may be at play.
The La Nina weather oscillation — which is the opposite of El Nino and tends to depress global temperatures slightly — ended. And the Arctic was unusually warm and had a record amount of sea ice melting — factors that alter weather throughout the Northern Hemisphere.
Meanwhile, some regions saw below average temperatures, including far eastern Russia, western Alaska, southern Africa, parts of the upper midwest and southeast United States, and much of China, the NOAA said.