The monarch, who is a passionate horse owner, was greeted with cheers as she rode into the famous racecourse in a carriage with the Prince of Wales and the Duchess of Cornwall.
Ascot hosts the famous five-day meeting which is as much a social event as an important sporting fixture that attracts some of the best thoroughbreds, jockeys and trainers in the world.
The Queen wrote in the official programme: "I was fortunate enough to own a winner at last year's royal meeting, so I know what a thrill it is to have success at this wonderful racecourse, whether you are a trainer, jockey, or one of the many hard-working stable staff associated with every runner.
"I would like to wish all those involved the very best of luck."
More top news
More than 1.85 million people have called for the US president's visit to be watered down to avoid causing "embarrassment" to the Queen.
Jim Mattis arrived in Baghdad on the day after US-backed Iraqi forces launched an operation to reclaim Mosul from IS.
The scale of the blood poisoning disease sepsis is "far worse" than previously thought thanks to a "crippling" lack of data, a study says.