The UN agency added that 2012 was the 27th consecutive year that the global average - which amounted to 14.45 degrees Celsius last year - was higher than the average between 1961 and 1990.
In annual report released Thursday, the WMO said that 2001 to 2012 were among the 13 warmest years on record.
The hottest year was 2010, when the mercury rose to an average of 14.6 degrees Celsius.
The Secretary-General of WMO, Michel Jarraud, says that the continued warming of the lower atmosphere is an ominous sign of global warming despite La Nina, which normally cools the world's oceans.
With files from the Canadian Press