The study from the Interactive Advertising Bureau said the economic value of online industries has roughly doubled since 2007 and constituted 3.7 percent of U.S. gross domestic product last year, up from 2.1 percent four years ago.
"The substantial economic impact of the Internet -- through its evolution and dynamism -- has been borne out by this study," study author John Deighton said in a bureau release.
New York and California were home to the headquarters of the largest number of U.S. Internet firms, the study found, while the jobs supported by the Internet include 375,000 in one-person firms and small companies, many of whom sell on Amazon, eBay and other sites, plus self-employed web designers, writers and programmers.
Digital industries directly employ 2 million Americans and online activity indirectly supports an additional 3.1 million jobs, the study found.
"The rapid growth of the ad-supported Internet has become a major driver in the U.S. economy," Randall Rothenberg, the bureau's president and chief executive, said.