The double Olympic champion, who won gold medals in the 800m and 1500m events in Athens in 2004, will come out of retirement for one day to take on 26.2 miles at the Virgin Money London Marathon.
On Monday, London Marathon organisers announced the 45-year-old will be among the 40,000 strong field of runners taking to the streets of the capital this April.
Britain's most successful female middle distance runner, Holmes still holds the British records for both 800m and 1500m and will no doubt give her first marathon a run for its money.
‘26.2 miles is a very long way for a long-time retired, middle distance runner,’ said Holmes, who retired from competition in 2005.
‘I will have to completely change my mindset to train for such a long distance but it’s a challenge that I am now up for, especially as for me it will all be about raising lots of money for charity.
‘It’s the most amazing event, the atmosphere is incredible and now I can’t wait until race day.’
Holmes is raising money for four charities, including the Dame Kelly Holmes Trust, Myeloma UK, the Pickering Trust and Hospice in the Weald.