The defendants admitted taking part in Operation Payback in December 2010 that targeted payment firms such as Paypal, Mastercard and Visa after they stopped processing donations to Wikileaks.
At least four UK youths were arrested on charges relating to the attacks on Paypal.
By pleading guilty, the defendants face relatively minor misdemeanour charges, as long as they stay out of trouble, according to the BBC.
Lawyers for the defendants argued they were taking part in protests that should be protected by the US Constitution, which guarantees free speech.
But, the US Department of Justice accused them of intentionally damaging a protected computer.
Read more about DDoS attacks
The DDoS attacks – made using a free tool downloaded from the internet called Low Orbit Ion Canon (LOIC) – were reported to have cost Paypal around £3.5m.
More than 100 workers from Paypal's parent company, eBay, spent three weeks working on issues related to the attacks.
PayPal also had to pay for more software and hardware to defend against similar attacks in the future.
Anonymous's Operation Payback originally targeted companies involved in the music industry and opponents of internet piracy. But the hacker collective broadened the campaign to include attacks in revenge for Wikileaks, following a backlash in the wake of the site publishing thousands of US diplomatic cables.