Normally men attending the horse racing event in the Royal Enclosure must wear black or grey morning dress with a waistcoat, tie and a top hat.
Women in the Royal Enclosure must not expose their shoulders.
Nick Smith of Ascot Racecourse said the jackets rule had not been enforced after entry due to the hot weather.
He said: "We took the decision early in the day, in the light of the hot weather, not to enforce the jackets rule after entry - common sense was applied."
The dress code at Ascot Racecourse's Royal Enclosure involves a number of restrictions for men and women.
Dresses must not be strapless, off the shoulder, halter-neck, spaghetti straps and or have straps narrower than 1in (2.5cm) in the exclusive area.
This year women were permitted to wear jumpsuits for the first time, as long as they reached to the ankle.
More on Royal Ascot 2017 from BBC Sport
However, midriff-baring outfits or fascinators are still not allowed in the exclusive area.
An Ascot spokeswoman said a relaxation of the dress code had "never happened before".
As temperatures soared at Royal Ascot today, race-goers tried to stay cool with sunglasses, parasols and fans.
Horses have been cooled down with "misters" after racing in the Unsaddling Enclosure
Up to 300,000 people are due to descend on the event over the next five days.
The Queen has attended Royal Ascot every year since she came to the throne in 1952.
Royal Ascot has not enforced its dress code, letting racegoers remove jackets for the first time in the event's history, due to 'very hot conditions'.