Sir Hugh said: "Ian Learmonth is a highly valued, hugely experienced colleague with a long and distinguished career.
"He has given unswerving service to policing, both within his own force and through his work with ACPO, more than four decades and his retirement will be keenly felt.
"I wish him and his family all the very best for the future."
Chief Constable Learmonth said: "Forty years' public service is a significant milestone, and a good time to reflect. The timing of retirement is not an easy thing to think about, but I have decided to do so in January. It has been a real privilege to lead Kent Police in the service of the people of Kent. The force has the best officers and staff I have ever come across - totally dedicated in their duty to protecting and serving the public. I leave with immense feelings of pride in what they have achieved in my three-and-a-half years as Chief Constable, and to have had the honour to lead Kent Police has been both humbling and inspiring."