Self-described "hacktivist" Christopher Weatherhead was convicted last month of conspiracy to impair the operation of computers. The attacks hit the sites of MasterCard, Visa, PayPal and several others.
Prosecutors said Weatherhead and his fellow conspirators targeted companies in the music industry involved in combating Internet piracy and companies that had stopped processing online donations to the secret-busting site WikiLeaks.
One co-defendant, 28-year-old Ashley Rhodes, was jailed for seven months. Another, 24-year-old Peter Gibson, received a suspended sentence.
The cybercrime spree focused attention on Anonymous, a loose-knit collection of Web-savvy activists and Internet pranksters.