Jo-ann Robertson became head of corporate comms and public affairs last April, having joined WS in 2003.
When Robertson joins Ketchum in September, she will become MD of public affairs and corporate comms. The title was previously held by Rod Cartwright, now director of its global corporate practice.
Jon McLeod, WS public affairs and corporate comms chairman, said: 'Jo-ann has been a smashing colleague.'
The agency will now recruit a separate head of UK public affairs and head of UK corporate comms.