Shoppers have spent $30.9 billion online from Nov. 1 through Dec. 16, up from $26.9 billion at the same point last year, said the Reston, Va., company, which tracks Web use.
Online sales surpassed $1 billion on four days last week. Total sales for the week climbed 15 percent to $6.31 billion compared to last year.
The five days that ended on Friday "will almost certainly be the heaviest week of the online holiday shopping season," said comScore chairman Gian Fulgoni. Online spending will begin to slow as Christmas draws closer, he said.
The holiday shopping season can make up to 40 percent of retailers' annual revenue. The online sales data point to Americans' growing comfort with using their personal computers, tablets and smartphones to shop for the holidays.
Discounting and promotions have also boosted shopping this year. ComScore said on Sunday that shoppers have received free shipping on at least half of all their purchases in each week of this year's holiday shopping season.