He will join the water-only company at the end of May once he has left Severn Trent, where he is currently the managing director of wholesale operations.
Cocks will succeed Richard Bienfait, who is standing down as chief executive at the end of the year, having announced his decision to leave in October.
In the interim, Affinity water’s executive team will be led by its chairman, Phil Nolan, who will act as executive chairman.
Nolan said: “ has extensive cross-sector experience and a strong track record in leadership roles in the utility sector, which will help us make the most of the opportunities ahead and deliver against our business plan commitments.”
On being named as the new chief executive, Cocks said: “It is an exciting time for the water industry and I am looking forward to taking up the role.
“Affinity Water has done an exceptional job in delivering an enhanced business plan and building an industry-leading position.
“I am really looking forward to leading the team to secure the successful delivery of the AMP6 business plan.”
Severn Trent’s recruitment process is “well underway” and a successor is due to be named in the New Year.