Brands2Life, the 25th-largest agency in the Top 150 for 2017, began work on the account on Monday.
The account was previously held by Text100, which continues to work with Darktrace in other European markets, but was won by B2L in a six-way pitch.
The new brief "sits towards the top end of clients in our business and technology practice in terms of size", according to André Labadie, deputy MD of the practice.
Darktrace's funding round this week puts its total value at £636m. It has more than 400 staff across 24 global sites. Darktrace's biggest backer is Mike Lynch, through his investment house Invoke Capital. Lynch was the founder of Autonomy, one of the biggest UK tech successes of recent times, having been sold to HP for £7.4bn in 2011. Autonomy and Darktrace were both founded in Cambridge, the latter in 2013.
Labadie said Darktrace was "seeing phenomenal interest for its self-learning approach to defending against cyber-threats" and that it wanted to "bring in a new agency to power the next stage of its growth through creative, impactful media relations in the very top tier media".
Assia Iossifova, PR director of Darktrace, said: "The Brands2Life team impressed us with their deep understanding of our domain and ability to cut through the noise and deliver impactful cyber stories that educate and inspire."