Don was lead presenter from 2003 until May 2008, when he left after suffering a minor stroke.
Toby Buckland and Alys Fowler have been dropped, as has the Birmingham Greenacre garden, which is only two years old.
Don will present the programme from his own garden in Herefordshire. He said: "Returning to Gardeners' World is a fantastic opportunity. I've really missed the programme so I can't wait to get stuck in and get my hands dirty again."
Alison Kirkham, Commissioning Editor, Factual Features and Formats, says: "I'm delighted to welcome Monty back and I really look forward to seeing him sharing his expertise in his own environment. I'm thrilled viewers will learn the secrets of the garden he's spent the last two decades creating from scratch."
Carol Klein and Joe Swift, plus former Gardeners' World presenter Rachel De Thame will also feature on the programme.