PHD will handle planning and buying for Siemens' global corporate campaigns. The expected spend in the UK is £5 million.
Local market planning and buying arrangements for the Siemens range of products are unaffected by PHD's appointment. MediaCom handles UK planning and buying for Siemens.
PHD's first task is to plan and buy a global campaign, created by Ogilvy, which will focus on health, industry and energy. UK activity will include press, outdoor and online.
Other key markets for corporate activity will be Germany, France, Italy, Brazil and the US.
The Omnicom network won the account - which has been dormant for the past five years - after presenting to the Siemens management team.
Lindsay Weedon, a managing partner at PHD London, said: "Siemens is a pioneering company with products that inspire and improve lives. This win is a tangible example of how the PHD network is geared up to run multimillion-pound campaigns."