The first six months of the year also set a record for warmth, according to the monthly report by the National Oceanic and Atmospheric Administration (NOAA).
"The globally averaged temperature over land and ocean surfaces for June 2015 was the highest for the month of June since record keeping began in 1880," said NOAA's National Centers for Environmental Information.
"The June average temperature across global land and ocean surfaces was 1.58 Fahrenheit (0.88 Celsius) above the 20th century average."
The previous high for the month of June was set in 2014, NOAA said. For the first six months of the year, 2015 beat out 2010 for the hottest on record.
The amount of Arctic sea ice was 350,000 square miles, or 7.7 per cent below the 1981-2010 average.
"This was the third smallest June extent since records began in 1979," NOAA said.