Grampian said that Knibbs, who has been in the role for two-and-a-half years, was leaving to pursue his career outside the business. Grampian said that it had no plans to fill the vacant position.
Knibbs joined Grampian in November 2003 in what was a new role for the company. He joined the executive board and was responsible for developing and implementing the company's marketing strategy.
Prior to Grampian he was commercial head of Leeds United Football Club. Previously, he was CEO for Bulmers North America and national field sales manager for Mars.
A spokesman for Grampian said: "During his time with Grampian, David has made a significant contribution to the marketing activities across the business. His clear focus has raised the added value agenda, providing a solid platform for the business to grow from."