Pitch will be responsible for Happy Socks' UK communications strategy, PR and influencer output, including an 'always-on' press office, creative activations and events centred on seasonal product launches.
Happy Socks was established in 2008 and its products are sold in more than 95 countries.
Pitch will drive awareness of the brand's hosiery, underwear and swimwear collections, and work alongside the internal team to deliver Happy Socks' brand vision.
Pitch Marketing Group director of PR and campaigns Chris Allen said: "They're a great addition to our growing roster of consumer PR clients and we look forward to creating some brilliant, feel-good work for them that delivers on their brand vision."
Happy Socks global PR and communications manager Sally Braid added: "Not only did Pitch demonstrate an excellent grasp of the Happy Socks brand vision, but also the chemistry, which is essential for us at Happy Socks when choosing partners."
Happy Socks is the latest addition to the agency's portfolio of lifestyle clients, which includes Beats by Dre, TAG Heuer and Asics.