The NAR said the index that tracks contracts of intention rose 6.7 percent month-to-month to 112.3.
The index is a comparison to the monthly average for 2001, its first year, which was assigned a value of 100.
The NAR said it had revised its April reading for the index to a slightly lower 105.2.
In May, the 25th consecutive month of improvements on an annual basis, the index was 12.1 percent higher than May 2012, when the index stood at 100.2.
"Even with limited choices, it appears some of the rise in contract signings could be from buyers wanting to take advantage of current affordability conditions before mortgage interest rates move higher," said NAR Chief Economist Lawrence Yun in a statement.
"This implies a continuation of double-digit price increases from a year earlier, with a strong push from pent-up demand," he said.
The Pending Home Sales Index for the Northeast held steady in May at 92.3, but is 14.3 percent higher than May 2012.
In the Midwest, the index, at 115.5, jumped 10.2 percent from April and 22.2 percent from May 2012.
From April, the index rose 2.8 percent in the South to 121.8, which is a gain of 12.3 percent over the past 12 months. In the West, the index came to 109.7 in May, up 16 percent from April, but up only 11 percent from May 2012 due to limited inventory of homes on the market, NAR said.