The group said sales rose in the month "despite headwinds."
Total sales of single-family homes, townhomes, condominiums and co-ops rose to a seasonally adjusted annual rate of 5.03 million in the month, from an upwardly revised 4.67 million in July.
The tally for August was 18.6 percent higher than the 4.24 million sales pace of August 2010.
NAR Chief Economist Lawrence Yun said, "Some of the improvement in August may result from sales that were delayed in preceding months, but favorable affordability conditions and rising rents are underlying motivations."
For August, the national median price for all housing types was $168,300, down 5.1 percent from August 2010.
In addition, for the month, foreclosure sales rose to 31 percent of sales, up from 29 percent in July, but down from 34 percent in August last year.
Total inventory of existing homes fell 3 percent at the end of the month to 3.58 million existing homes for sale, a 9.5 month supply at the current sales pace.