Nov 1 (Reuters) - U.S. online spending rose 9 percent to $32.1 billion in the third quarter, market research firm comScore (SCOR.O) said on Monday.

Amazon.com

It was the fourth straight quarter of year-over-year growth following the U.S. economic downturn, comScore said.

In the 2009 third quarter, online sales fell 2 percent.

ComScore Chairman Gian Fulgoni called the data "a fairly positive indicator for the upcoming holiday season," but he cautioned that continued high unemployment could crimp consumer spending.

"Until the economy begins adding jobs at a meaningful rate, the lack of spending power among consumers will continue to be a drag on purchasing, with many consumers indicating their intention to cut back on gift buying this holiday season," Fulgoni said in a release.

The best-performing categories during the quarter were books and magazines, excluding digital downloads, computers and consumer electronics.

Growth in online shopping, which makes up just a fraction of the overall retail pie, has outperformed brick-and-mortar sales growth during the downturn and the aftermath.

Last month, comScore forecast that e-commerce spending during the holiday season could rise 7 percent to 9 percent if unemployment does not worsen.

The National Retail Federation, meanwhile, has forecast that holiday retail sales, which exclude online sales, will rise 2.3 percent to $447.1 billion.

Online giants Amazon.com (AMZN.O) and eBay Inc (EBAY.O) are battling for market share with websites run by brick-and-mortar stalwarts such as Wal-Mart Stores (WMT.N) in advance of the holiday season, which can account for as much as 40 percent of annual sales.

Tepid growth in consumer spending has meant more competition, and comScore found that the top 25 online retailers accounted for 70 percent of dollars spent in the third quarter.

ComScore said 41 percent of online transactions in the quarter included free shipping, a key perk given by retailers to inspire purchases.

(Reporting by Alexandria Sage; editing by John Wallace)