Fullsix will service the account from London while continuing its eight-year relationship with Hertz in France.
The agency won the account for its "creativity and innovation, strong marketing strategy and robust European delivery network".
Hertz began seeking a digital agency in May when the brand drew up a shortlist of agencies to work on a web-build project and online marketing. One of the other agencies named was Global Beach.
Steve Baer, managing director of Fullsix UK, said: "Our core strength of creating and managing global digital marketing programmes across the Fullsix network will help to deliver Hertz's strategy across Europe."
Rhys Jarvis, Hertz head of online marketing and strategy, said: "Fullsix is one of the leading agencies in the sector and brings to the account a strong pan-European network."
DDB London holds the creative account for Hertz, while DDB WWAV Rapp Collins handles direct marketing for the company.
The DDB London appointment in November last year followed on from less than a year relationship between Hertz and Rainey Kelly Campbell Roalfe/Y&R.
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