Home sales dropped 3.2 percent to a seasonally adjusted annual rate of 5.12 million homes, the trade group said.
It was the second consecutive month of declines after the index established a four-year peak in August.
From a year earlier, sales rose 6 percent. In October 2012, sales at were at an annual rate of 4.83 million homes on a seasonally adjusted basis.
NAR also said the median price of a home in October, at $199,500, is 12.8 percent higher than the median sales price in October 2012, marking the 11th consecutive month in which the median prices rose in double digits from the same month a year earlier.
The trade group said the median time on the market for an existing home came to 54 days in October, up from 50 days in September, but down considerably from the median of 71 days on the market in October 2012.
At the current rate of sales, there is a 4.9-month supply of homes on the market with unsold inventory 0.9 percent above a year earlier, when the supply of listed homes represented a 5.2-month supply.
While the data is mixed, "the erosion of buying power is dampening home sales," said NAR Chief Economist Lawrence Yun.
"Moreover, low inventory is holding back sales while at the same time pushing up home prices in most of the country. More new home construction is needed to help relieve the inventory pressure and moderate price gains," Yun said in a statement.