The firm had contacted agencies earlier this year about the brief, and has decided to end its four-year association with Text100. The PR Network begins work on 1 April.
Text100 has also this month decided to end its 17-year relationship with tech titan IBM, and had its work with Vodafone significantly reduced in scope recently. Elsewhere, it has won a new brief with tyre maker Bridgestone.
Text100 UK MD Tara O'Donnell said: "After four successful years partnering with Sophos, Text100 decided not to repitch. Both companies are proud of their shared accomplishments in building awareness for Sophos in the UK."
Sophos' review was led by new senior EMEA PR manager Louise Harris, who joined late last year from software firm Kronos.
She said the agency would be "focused primarily on b2b engagement", working alongside UK and Ireland PR manager Sophie Pithie, adding: "As our product portfolio expands into the consumer security space, with Sophos Home Premium, The PR Network will also work to drive awareness and engagement with the end consumer around issues relating to cybersecurity protection."
Harris also said: "The virtual nature of The PR Network structure was a real differentiator for us, as it will allow us to engage relevant experts to support projects in different verticals and product areas."
The agency's co-founder George Blizzard said: "This is yet another testament to our virtual PR model, which allows forward-thinking brands like Sophos to work with handpicked senior and expert teams."
Sophos, which listed in 2015, uses Tulchan for financial PR. Tulchan took over from Brunswick last year.