This September was about 0.78 degrees Celsius above the 1951-80 average temperature for the month, data from NASA's Goddard Institute for Space Studies (GISS) has showed.
This makes it even more likely that 2014 will become the warmest year on record, Climate Central reported.
The new NASA data makes it the warmest September in the GISS records, edging out the previous September record set in 2005.
"If we continue a consistent departure from average for the rest of 2014, we will edge out 2010 as the warmest year on record," Jake Crouch, climatologist with the US National Climatic Data Centre, was quoted as saying.
September followed the warmest August on record.
Ocean temperatures have played a large role in this, including the warmer-than-normal sea surface temperatures in the tropical Pacific Ocean that have accompanied an emerging El Nino, the report added.