The pitch process, which came down to a shortlist of four agencies, was handled by the AAR.
Rias was established in 1992 and offers a product range including home, motor, travel and pet insurance. It has more than 880,000 customers.
It has given WPN responsibility for developing a strategy for all customer acquisition, customer communications and brand development. The agency will begin work immediately to create a direct mail, inserts and direct response press ad campaign.
Andrew Kennett, head of customer acquisition at Rias, said: "Our business faces some ambitious growth plans with challenging targets and we have chosen an agency to help us develop the strategy to differentiate RIAS and deliver future success.
"WPN demonstrated a real appreciation of our opportunities, showed us some compelling creative work and completely convinced us they were the team for us."
The £5m media planning and buying account for the insurer is with Tri-Direct.