He replaces Hamid Habib, who last week joined MPG Media Contacts as the head of its strategy and research unit, Catalyst.
Shah, who will report to Zenith Media's managing director, Stephen Farquhar, will be responsible for driving strategy across clients and ensuring that the agency is pushing its communications planning further.
David Grainger, the head of strategy at its sister agency Optimedia, will continue in the same role.
Shah started his career in 2005 as a digital planner at the now-defunct Zed Media, before becoming a communications planner at ZenithOptimedia in 2007.
Most recently, Shah led the strategic effort on Zenith Media's successful pitch for RBS Group.
Farquhar said: "Rian is one of the strongest strategic thinkers in the industry."