The man leading the project to connect Birmingham with London via high-speed rail is leaving to join Rolls-Royce.
He will take up the newly created position of chief operating officer in the coming months with the renowned manufacturer of engines for the aerospace, marine and energy sectors.
Mr Kirby, who joined HS2 in May 2014 having previously been on the board of Network Rail, said: "HS2 is not just a highly ambitious project but also one which will leave a lasting legacy for Britain.
"It has been a huge honour to have been its chief executive and to have been involved in creating a leadership team made up of the best talents from this country and elsewhere.
"I have absolute confidence in their ability to deliver the project and, in doing so, to help transform the way we do things in this country."
HS2 chairman Sir David Higgins added: "I am delighted for both Simon and Rolls-Royce that he has been appointed to this position.
"While naturally we will miss his experience and leadership, I also recognise that he is joining a truly great, global company in an industry in which he has previously worked."
In a separate announcement, the former chief executive of Amey Mel Ewel, the company with the 25-year road maintenance contract for Birmingham, has been appointed as a non-executive director to the HS2 board.