Many died while wandering through vast desert areas where summer temperatures regularly soar past 40C (104 Farenheit).
In one Arizona county, a refrigerated trailer is being used to store up to 70 extra bodies as the morgue is full.
Temperatures in the western US desert regions have remained unceasingly "brutal" throughout the summer, Border Patrol spokesman Jose Garza told the Reuters news agency.
"The smugglers continue to bring these people to the vastness of our western deserts... and we cannot control the elements."
Temperatures in Arizona have peaked at 44C this year, with the mercury in Tucson staying at 40C or above for 29 consecutive days in June and July. An estimated 81 immigrants died during that heatwave.
Arizona serves as the main crossing point for some 1.1 million immigrants who make the journey into the US from Mexico each year. Border Patrol officers have carried out about 700 missions to rescue immigrants stranded in the desert region this summer.