It comes as Fisch targets Europe as a key part of its 2015 growth plans.
Instinctif has previously represented Fisch from its Zurich office for a more limited brief. The agency inherited the account from blueRobin, the Swiss firm it acquired last year.
It is understood there was no competitive pitch.
Ben Curson, Instinctif managing partner, capital markets and corporate affairs, said: "It is a very exciting company with a unique offering, which we believe will be welcomed by the European markets. We look forward to working closely with the excellent in-house marketing team to boost Fisch’s profile in Europe as it continues its rapid growth into 2015."
Hansjoerg Herzog, Fisch head of marketing and sales, said: "We are ready to expand our strategic communications programme, and we believe that Instinctif will be the ideal partner to see this initiative achieve its full potential."