Course bosses made the decision following an 8am course inspection and today's racing has been cancelled due to a waterlogged track.
However, officials are confident the rest of the meeting will go ahead over the weekend.
Today's racing would have kick started the first of the three-day meeting which is a highlight of the flat racing season in Scotland.
The main race of the opening day would have been the Williamhill.com Handicap for the Kilkerran Cup – one of the top middle distance handicaps in Scotland, with £20,000 in prize money.
The decision to abandon racing comes ahead of tomorrow's visit by the Duke and Duchess of Rothesay.
A spokesman for Ayr Racecourse said: "It is devastating. We had 9mm of rain overnight and the forecast is for another 8mm until 6pm this evening.
"There is standing water on the track which meant the racing had to be abandoned today.
"The forecast for later is drier and sunnier spells and we are confident that racing on Friday and Saturday would go ahead."
A prolonged period of rainfall overnight saw around 4mm fall in a couple of hours.
Racegoers are being offered a refund or a chance to use their tickets tomorrow.
People with hospitality tickets are being offered a refund or an alternative date at a future race meeting.
Royal guests the Duke and Duchess of Rothesay, Charles and Camilla, will unveil a plaque renaming the Paddock Stand, the Rothesay Stand when they visit the course as guests of honour at the William Hill Supporting Dumfries House Race Day, which is Ladies' Day.
The Duke and Duchess will present the trophy for William Hill Supporting Dumfries House Ayrshire Handicap Stakes, the feature race of the afternoon.