The transaction, which values Sophos at $830m, will see Sophos founders Jan Hruska and Peter Lammer retain a minority shareholding and remain as non-executive directors.
TA Associates, a minority shareholder in Sophos since 2002, will sell its full interest to Apax in this transaction.
Sophos, founded in 1985, claims 100 million users in more than 150 countries, with customers including Cisco, Marks & Spencer, Heinz and Harvard University.
In its fiscal year ended 31 March 2010, Sophos reported revenue of more than $260m, compared with $213m the previous year, and generated free cash flow of $55m, up from $40m the year before.
"As the market continues its migration from point solution to tailored, unified security suites, Sophos's strategy to offer the world's most resilient, cost-effective solutions without any additional complexity remains key", said Steve Munford, chief executive of Sophos.
Apax's financial backing, combined with Sophos's understanding of security and data protection is great news for our customers, prospects and partners, he said.
"We identified the security software space as an attractive investment area for us given its rapid growth driven by ever increasing malware threats and high barriers to entry. Sophos is a very strong platform and is gaining market share", said Salim Nathoo, a partner in the tech and telecom team at Apax Partners.
Apax's strong track record and industry specific knowledge in the technology sector makes Sophos a perfect fit, he said.
This article was first published by Computer Weekly