The appointment follows a four-way pitch process led by Zurich PR consultant Tracy Dickerson in October.
The brief includes elements of consumer and corporate communications, with digital and "social 'as standard'", and will "reflect the brand's ambition to transform the way individuals, businesses and intermediaries view Zurich".
The Third City account team, led by managing director Chris Blackwood, will report to Zurich UK head of corporate affairs Sophie Timms (pictured), with the aim of bringing to life Zurich's "excellent culture, leading sustainability credentials and highly regarded products and service".
"As an insurer first and foremost, the touchpoints we have with customers and other stakeholders are incredibly wide-ranging. As a business, we are passionate about maintaining a positive impact on society, communities and employees," Timms said.
"This means our communications strategy runs so much deeper than just a 'shop window'. Our mission is to build a brand that people really want to connect with on several levels. We were overwhelmed by the quality and talent of the communications professionals we met during the pitching process."
Timms said Third City stood out due to the way the agency "embraced the brief".
Blackwood said: "We want to take the nation on the same journey that we've been on, so that they discover the fantastic and progressive initiatives Zurich leads. And in an industry so often overly focused on product, Zurich's strategic approach to the wider brand is refreshing, suited to Third City's 'corpsumer', channel-neutral approach."