For the most recent week ending Dec. 17, online sales hit $5.5 billion, an increase of 14 percent compared with the same week last year.
Four individual days during the week surpassed $900 million, led by "Green Monday" on Dec. 13 at $954 million, an increase of 12 percent, and "Free Shipping Day" on Dec. 17 at $942 million.
"Free Shipping Day" saw the largest percent change, with a 61 percent increase in sales over last year, up from $586 million. More than 1,500 retailers offered free shipping.
"While no individual days during the week surpassed $1 billion in spending, we saw strength throughout the week," comScore chairman Gian Fulgoni said in a statement.
The National Retail Federation revised its holiday sales forecast last week to 3.3 percent, or $451.5 billion, up from the initial estimate of 2.3 percent.
For the most tracked shopping days of the season, "Cyber Monday" led the way, with more than $1 billion in sales, an increase of 16 percent over last year.
Thanksgiving Day online sales were up 28 percent to $407 million, and "Black Friday" improved by 9 percent.
"At this late juncture in the online holiday season, we have likely already witnessed the peak spending day of the year, which means that Cyber Monday should emerge as the season’s heaviest online spending day for the first time in history," Fulgoni said.
The research firm said that computer hardware, including Apple's iPad and laptop computers, drove most of the growth. Flat-screen televisions also helped along online sales.