The appointment is key to Huffington Post's investment in investigative reporting, unveiled last March and which has a $1.75m investigative fund.
The hiring was announced by Ms Huffington, the co-founder and editor-in-chief who chairs the fund's advisory board, and Nick Penniman, executive director of the fund.
Huffington said: "Larry's background in investigative journalism -- including being part of three Pulitzer-winning teams -- as well as his experience in online journalism makes him an ideal pick to spearhead the investigative fund.
"Plus his experience as a business editor is perfectly aligned with the investigative fund's initial focus on covering the economic crisis."
Roberts joined The Washington Post in 1995 as one of the founding editors of its website. He has overseen the title's investigative coverage since 2004 and directed coverage of the Abramoff lobbying scandal and a series of pieces on then-US vice president Dick Cheney, both of which won Pulitzer prizes.
The Huffington Post's launch of its investigative fund was partly in response to the job cuts and closures across the US newspaper industry.
It is headquartered in Washington DC and is aimed at producing a range of investigative journalism stories -- from long-term pieces to short breaking news stories.
Huffington's first hiring for the fund was that of Jai Singh in April. Singh, the founding editor-in-chief of CNET, will join the site as managing editor.