PRWeek understands that the enterprise search firm approached 30 different agencies to pitch for the account, having dropped its retained agency of three years, Bite PR (PRWeek, 11 July).
David Vindel, Red's head of technology, will lead the account.
Neither Red nor Autonomy would comment on the news.
Autonomy is listed on the FTSE 250 and is the second largest firm in Europe that deals purely with software. It has been described recently as 'Britain's biggest technology success story' and names the BBC, Coca-Cola and NASA among its clients.
Autonomy will be the Red Consultancy's biggest business-to-business account. One source suggested that Red's experience in Autonomy's sector might have been crucial in the company, which regarded Red as a good fit.
Since Vindel joined in January this year, The Red Consultancy has won accounts for the internet security company McAfee and the technology manufacturer Epson.
Cambridge and San Francisco-based Autonomy's customer base comprises more than 17,000 firms including AOL, Deutsche Bank and the New York Stock Exchange.