Harry had the prestigious title removed by the Queen in 2020 after he and Meghan announced they would step down as working royals and move to California.
There had been speculation about which member of the Royal Family would be given the role, with Princess Anne tipped for the position.
The Duke of Sussex was appointed Captain General by the Queen in December 2017, succeeding Prince Philip.
Today’s announcement comes just a day after Harry unveiled his ‘provocative’ new memoir Spare, a ‘loaded’ reference to his position as the younger brother of the heir to the throne.
In a personal message to the Royal Marines as they mark their 358th birthday today, the King said: ‘It is the greatest possible pleasure to assume the role of your Captain General. I am exceptionally proud to follow in the footsteps of so many members of my family over the last three-and-a-half centuries, all of whom held the role with a deep sense of admiration.
‘The Royal Marines have a distinguished and unparalleled history, both on land and at sea. I draw immense inspiration from your courage, determination, self-discipline and a remarkable capacity to endure in the most extreme environments.
‘I feel greatly honoured to become part of the Corps Family and very much look forward to meeting many of you in the near future. In the meantime, this comes with my heartfelt and special wishes for a very happy 358th birthday. Per Mare, Per Terram.’
In 2017, Harry had succeeded the Duke of Edinburgh, who had held the post for 64 years before standing down following his retirement from public duties.
However, in February last year, Harry and Meghan were stripped of their prestigious patronages as the couple confirmed Megxit was permanent following a 12-month review of the Sussexes’ decision to quit as working royals.
The duke also lost his other formal roles with the military including Honorary Air Force Commandant of the Royal Air Force Base Honington, and Honorary Commodore-in-Chief of the Royal Naval Commands’ Small Ships and Diving.