Wales Pension Partnership, Carmarthen, hired Robeco as voting and engagement provider.
Robeco will help the £17.5 billion ($22 billion) pool of local government pension schemes formulate and maintain voting and engagement policies for a period of three years.
Robeco will subsequently implement these policies across WPP's £5 billion active equity portfolio, a news release said.
"The appointment of Robeco is evidence of the WPP's continued delivery of the commitments made in its responsible investment policy and its vow to exercise its voting rights in line with the interest of its stakeholders, engaging with investee companies to enhance the long-term value of the constituent authorities' investments within WPP," said Peter Lewis, chairman of the joint governance committee at the pool, in a separate news release.
WPP will publish its voting policy and engagement principles on this website once they are finalized. The pool can extend the contract with Robeco once for additional two years.
A spokesman could not be reached to provide further details.