Police chiefs in South Yorkshire stress the operation is not in response to any particular threat.
The St Leger Festival, which starts today, is a key event in the racing calendar and looks set to attract around 30,000 on Saturday.
Festival goers will be searched on entry, there will be armed officers on the gates and a search dog will be used.
A South Yorkshire Police spokeswoman said: "We want everyone who goes to this year's St Leger to have a fantastic time. But our priority has to be keeping everybody safe.
"We are not responding to any particular threat to public safety. However, during the festival our officers and racecourse staff will have an enhanced security operation in place.
"If you are going to the festival, you will be searched before you go in. Please bear with us and allow enough time to get into the venue.
"We will also be using a search dog and you can expect to see armed offices at the entrance points. This is for your reassurance. Again, is not a response to a particular threat.
"We are also asking you to be vigilant while you are at the festival and to report anything suspicious to one of our officers or a member of the security staff. If you see or hearing something that concerns you, please let us know."