The incumbent on the account for nine years, Publicis, pulled out of the pitch at an earlier stage. A number of financial specialist agencies were involved in the contest, prompted by a statutory review called in October.
Masius will produce the creative for the key first quarter of the year, when most people make their ISA and self-invested personal pensions (Sipp) investment decisions.
Chris Dell, managing director of Masius, said: "We're delighted with the win — it really is an interesting time to be working with Fidelity helping promote their business. This appointment, along with other recent wins, helps cement our agency at the heart of the new financial services landscape."
Fee income on the account is understood to have dropped dramatically as the investment sector experienced a drying up of fund inflows, which turned to outflows in some months in 2007 and 2008, though 2009 has seen a return to much healthier levels, according to Thomson Datastream.
Will Arnold-Baker, managing director of Publicis London, said: "We stuck with Fidelity in 2009 during a time of vastly reduced spend but it's probably not a sustainable position for us going forward. Now feels like the right time to part company."