Williams has been CFO since 2015 at Pearson, whose shares have plunged nearly 50% since early 2019.
At Adecco Group, he will succeed Hans Ploos van Amstel, who is leaving in June after almost five years.
"I am looking forward to .... building on the great work of Hans to further advance the group's transformation, harness the significant opportunities ahead to drive profitable growth, and deliver clear value for stakeholders," Williams said in a statement.
Adecco wished Ploos van Amstel well, saying he had helped improve the company's cost structure, adjust its portfolio and roll out digital offerings.
Williams' departure from Pearson was announced on Thursday alongside a profit warning that sent its shares to their lowest level in more than a decade.
It comes a month after CEO John Fallon said he would retire this year.
The British education group has been caught off guard by the speed of the switch from physical textbooks to digital products, resulting in a string of profit warnings.
Williams will be succeeded at Pearson by his deputy, Sally Johnson, Pearson said.
(Reporting by Michelle Martin in Berlin and Paul Sandle in London; Editing by Michael Shields and Jason Neely)