The agencies, Brand Union, The Partners, Lambie-Nairn, Addison Group and Vbat, will be combined to form a new agency, which has not yet been named.
The new agency will be led by The Partners and Lambie-Nairn chief executive Jim Prior as global chief executive. Simon Bolton, the worldwide chief executive of Brand Union, will be executive chairman.
The moves comes amid pressure on WPP chief executive Sir Martin Sorrell to restructure and simplify WPP in response to slowing revenue growth and changing client needs, according to people inside and outside the company that have spoken to Campaign.
WPP is currently merging two of its Group M agencies, MEC and Maxus, as well as moving Possible and Salmon into Wunderman.
The world’s biggest holding company has suffered from cuts by FMCG companies and reported a 1.7% slump in net sales in the second quarter of 2017. WPP cut its annual growth forecast to "between zero and 1%".
In a statement, WPP said the decision to bring the agencies together is part of a "client-centric plan" for growth, allowing clients to be able to access a wider array of specialists.
The new agency will launch in January 2018. WPP said it will have a network of 750 people working across 20 countries, with client billings of more than $100m (£74.8m).
Prior said: "Our clients and our industry are ready for change and by bringing these agencies together, we can serve clients across the full range of sectors, capabilities and geographies.
"This convergence builds the next generation brand agency and is motivated by the opportunities for growth—for our clients and for us."