He leaves the role with immediate effect. Paul Taaffe will be responsible for finding Pedersen’s successor when he takes up his role as Huntsworth chief executive in April, replacing the retiring Lord Chadlington.
"In the meantime, the executive management team of Grayling will manage the division on a day-by-day basis," said Huntsworth in a statement.
Chadlington said: "Over the past two years, Pete Pedersen has made an important contribution to Grayling and its repositioning as a digitally focused international PR company building multinational client engagements. We wish him the very best in the future."
Pedersen tweeted about his resignation this afternoon:
Before joining as Grayling CEO, Pedersen worked at Edelman, most recently chairing its global technology practice.
His resignation follows a tumultuous period at Huntsworth, which has seen the resignation of CEO Lord Chadlington and chairman Lord Myners and the appointment of their successors, Paul Taaffe and Derek Mapp respectively.
Huntsworth issued a profit warning in July and interim results in August showed revenue falls at Grayling, Citigate and Red, offset by growth at Huntsworth Health.