The trade group attributed December's decline partly to "abnormal weather" that slowed home shopping "in much of the United States."
The trade group's Pending Home Sales Index fell 8.7 percent in the month to 92.4 from a downwardly revised 101.2 in November.
The index is 8.8 percent lower than the December 2012 reading of 101.3.
"Unusually disruptive weather across large stretches of the country in December forced people indoors and prevented some buyers from looking at homes or making offers," said NAR Chief Economist Lawrence Yun.
"Home prices rising faster than income is also giving pause to some potential buyers, while at the same time a lack of inventory means insufficient choice," he added.
NAR said the pending home sale index fell in each of the four regions of the country, dropping 5.5 percent below December 2012 in the Northeast, 6.9 percent in the Midwest and the South and 16 percent in the West.