Official figures show the United States is now in the middle of the hottest year since record keeping began in 1895.
The National Oceanic and Atmospheric Administration also said the US had broken its record for the hottest six months in any calendar year.
According to the agency, more than 170 hot weather records were broken or tied during the second half of June.
However, the BBC reports June was still only the 14th hottest on record - the hottest being June 1933, during the Dust Bowl period.
A recent 11-day heatwave has claimed at least 46 lives. However, a cold front moved in to the Midwest and East Coast late on Sunday.