Tokyo 2020 organisers announced the rescheduling on Wednesday, following discussions with the International Olympic Committee and World Athletics, to ease the burden on National Olympic Committees and athletes travelling to northern Japan.
It now means that all the marathon and race walk events are scheduled to be held on four consecutive days, from August 6 to 9, with the men's marathon remaining on the final day of the Games, as per tradition.
Both marathons will start at 7.00 am local time (2200 GMT). The women's marathon was initially set for August 2.
The IOC stunned Games organisers in October by announcing the marathon, one of the most prestigious events at any Olympics and one especially beloved in Japan, would be moved to the northern Japanese city of Sapporo to avoid the worst of Tokyo's summer heat.
Tokyo temperatures in July and August, when the city will host the Games, regularly exceed 30 degrees Celsius with high humidity adding to the discomfort.