Pitt joined McAlpine nearly fours year ago from Morgan Sindall after a management shake-up ending a 17-year career with the firm.
He is understood to have left after a boardroom disagreement.
Pitt will work out his notice period working alongside the business in order to help close out a critical commercial project.
A spokesman for Sir Robert McAlpine said: “Martin Pitt has decided it’s time for him to move on from Sir Robert McAlpine in order to pursue new challenges and opportunities.
“Martin has stepped down with immediate effect as Commercial Director. Leighton More, chief financial officer will assume responsibility for all commercial matters whilst the business reviews its needs in line with our five-year strategic objectives.”