The price tag, which is thought to be as high as £15m ($30m) includes an earn-out, which will depend on the future performance of the company, according to Kara Swisher's BoomTown blog.
ContentNext, which was founded by publisher and editor Rafat Ali in 2002, is an independent media company with offices in Santa Monica, California and New York.
It operates the highly rated paidContent.org and its UK version paidContent:UK, as well as mocoNews.net, which covers the business of mobile content, and contentSutra.com, which covers India's digital content market.
ContentNext also runs several conferences and is headed up by chief executive Nathan Richardson.
Ali and Richardson will continue to run the company, which is based in Santa Monica, California, and New York, as a stand-alone business. ContentNext will join the GMG's B2B business Guardian Professional.
According to Ali: "The Guardian's international reputation for editorial integrity and digital innovation make it the perfect partner to help ContentNext to grow its expanding platform.
Tim Brooks, managing director of Guardian News and Media, said: "We have long been admirers of Rafat and the business he has built, which is an indispensable resource for so many senior people in our industry. So we are genuinely excited at the prospect of being able to help Rafat, Nathan and the team take ContentNext to the next level."
Richardson said: "The strategy and ambitions of the Guardian are a terrific fit for ContentNext’s plans to be the leading digital publishing platform for news and information to the media, entertainment and publishing industries."
The move marks a major step in Guardian News & Media's expansion outside of the UK and follows on from the launch last year of Guardian America, which offers news and comment to a US audience. GMG has also formed commercial partnerships with Reuters to sell advertising in the US.
Carolyn McCall, chief executive of Guardian Media Group, said: "I have admired paidContent for years. Rafat and his team personify the values of editorial independence and integrity that are core to the Guardian. I'm very happy to welcome the company into the expanding Guardian Media Group."