The agency won the business after a pitch against Albion, VCCP and a team from Karmarama and Kitcatt Nohr Alexander Shaw.
Arc, the incumbent on the account for the past three years, declined to repitch.
The review process, which began in September last year, was handled by Rik Andrews, the head of marketing for Europe for iShares, and the consultancy Oystercatchers.
M&C Saatchi will now be responsible for creating a series of integrated campaigns to help build the iShares brand across six European markets - the UK, Italy, France, Benelux, Switzerland and Germany.
Work will incorporate TV, digital and direct marketing activity, and will at first aim to raise awareness about the benefits of exchange-traded funds to a wider audience.
Earlier this year, Barclays Global Investors, the owner of iShares, sold the business to the global investment management firm Blackrock in a mixed cash-stock deal worth almost £13.5 billion.