The Huffington Post, which began in 2005 with a meager $1 million investment and has grown into one of the most heavily visited news Web sites in the country, is being acquired by AOL in a deal that creates an unlikely pairing of two online media giants.

The two companies completed the sale Sunday evening and announced the deal just after midnight on Monday. AOL will pay $315 million, $300 million of it in cash and the rest in stock. It will be the company’s largest acquisition since it was separated from Time Warner in 2009.

The deal will allow AOL to greatly expand its news gathering and original content creation, areas that its chief executive, Tim Armstrong, views as vital to reversing a decade-long decline.

Arianna Huffington, the cable talk show pundit, author and doyenne of the political left, will take control of all of AOL’s editorial content as president and editor in chief of a newly created Huffington Post Media Group. The arrangement will give her oversight not only of AOL’s national, local and financial news operations, but also of the company’s other media enterprises like MapQuest and Moviefone.