The consumer goods giant has been seeking to slash agency fees and the number of agencies with which it works globally.
However, a source familiar with Unilever’s thinking said it wants to have better co-ordination between global and local media planning and maintained the account switch was not a cost-cutting move.
An industry source estimated Omnicom’s PHD employed in the region of 30 people on Unilever’s global communications planning, which could mean the account is worth about $4m to $5m in fees.
PHD won the global communications planning account from Mindshare in 2012 after a competitive review.
WPP’s Mindshare already manages much of Unilever’s media-buying globally, including the US.
PHD also handles some of Unilever’s media-buying in a number of markets such as Australia and parts of Asia and that is unaffected by the shake-up to global communications planning.
A Unilever spokesman said: "In line with the digitisation and fragmentation of today’s consumer and media as well as our Connected 4 Growth programme, which is giving Unilever more local agility and speed, we are evolving our media agency arrangements.
"We will be partnering globally for communication planning with Mindshare to increase our effectiveness and deliver greater global-to-local relevance, future-proofing our marketing communication in a rapidly changing media landscape.
"We would like to acknowledge the value that PHD have delivered to our business and will continue working in markets with our current agency partners PHD, Initiative and Mindshare for local media planning and buying."
Unilever, the owner of brands such as Dove, Hellmann's and Magnum, promised to slash costs at the start of this year after fending off an unwanted takeover bid by Kraft Heinz.
The Anglo-Dutch company said it would halve the number of agencies that it uses from 3,000 and reduce the number of ads it makes because it has been over-saturating the market and not "wearing" them out.
However, Unilever has insisted that it will not reduce its media spend. The company spends close to €7.5bn a year on brand and marketing investment.
Unilever told shareholders at a two-day investor meeting this week: "Supply chain savings and zero-based budgeting programmes are delivering faster than planned.
"These actions enable reinvestment behind our brands for growth, and provide momentum towards our underlying operating margin target of 20% by 2020."
The share price has leapt by more than 40% this year after the move to cut costs and streamline the group.
WPP, Denstu Aegis Network and Havas are among the groups which have blamed cost-cutting by FMCGs for a slowdown in revenue growth this year.