The $250 million deal closed Tuesday, starting a new chapter in the history of the storied newspaper.
It is owned, at this point, by Nash Holdings, which is Bezos's investment firm, the newspaper said.
For the past 80 years, the Post had been owned by the Graham family.
In August, Chief Executive Officer Donald Graham said Bezos would be an asset to the newspaper. He has a sufficient amount of capital to keep the business going and experience marketing an Internet firm, Graham said.
Graham also noted Bezos appears to be entering the business without a strong ideological bias, Graham said.
Bezos said in September he has the resources to allow the Post to survive while the management team searches for a profitable formula for the business that has been losing money due to waning advertising revenue and readers who are turning to the Internet for news.
Bezos said he would keep his "day job" at Amazon and retain Post publisher Katharine Weymouth, Graham's niece, and executive editor Martin Baron.
"We are officially under new ownership, and a new era for The Washington Post begins," Weymouth wrote in a note to employees Tuesday.
She also said the newspaper would build on its tradition as "a standard-bearer for world-class news reporting and analysis."
"With Jeff's leadership, we will build on that foundation and take the Post to new heights," she wrote.
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