Following a competitive pitch against eight other agencies, 33Seconds will lead content creation and community management across the car maker's social platforms, for a period of three years.
Prior to winning the pitch for all three brands, 33Seconds had been Peugeot's retained UK social media agency since 2014.
Dominic Cook, co-founder and CEO of 33Seconds, said: "We've led some exciting and innovative campaigns for Peugeot over the last couple of years - including #208Unleashed and our more recent Just Add Fuel campaign - which achieved impactful results. We're looking forward to replicating this success for the Citroën and DS teams."
Stephen Murray, a senior account manager at the agency, told PRWeek the key objectives of the brief were to continue raising awareness of all three brands and to drive leads.
Groupe PSA joins Amazon, Magners, Greenpeace, SoundCloud and Web Summit as part of 33Seconds' current portfolio of clients.