The purchase by Bezos, who is Amazon.com's chief executive, will end four generations of stewardship of the 136-year-old newspaper by the Graham family, the Post noted. It's expected to be completed within a couple of months.
The Post said Bezos will be the sole owner with no involvement by Amazon.com.
The Washington Post Co., which has other media holdings, will change its name and remain a publicly traded company, the newspaper said.
The sale announcement caught just about everyone by surprise, the Post reported.
"Every member of my family started out with the same emotion -- shock -- in even thinking about" selling the Post, Donald Graham, the parent company's chief executive said. "But when the idea of a transaction with Jeff Bezos came up, it altered my feelings.
"The Post could have survived under the company's ownership and been profitable for the foreseeable future. But we wanted to do more than survive. I'm not saying this guarantees success but it gives us a much greater chance of success."
The company's newspaper division has endured a 44 percent decline in operating revenue in the past six years and its circulation continues to atrophy -- 7 percent so far this year.
Bezos, 49, told the Post he considers the newspaper "an important institution" and while expressing optimism indicated he had no pre-conceived notions.
"I don't want to imply that I have a worked-out plan," he said. "This will be uncharted terrain and it will require experimentation.
"There would be change with or without new ownership. But the key thing I hope people will take away from this is that the values of the Post do not need changing. The duty of the paper is to the readers, not the owners."