The ITV costume drama is filmed at Highclere Castle in Berks, from which HTR takes its name.
Mr Bonneville, who plays Lord Grantham in the popular series, is now a part-owner of two race horses in one of the Highclere syndicates.
He said “I am thrilled to become an owner with Highclere. I thought it was about time I became properly educated in the Sport of Kings.
“It’s a terrific opportunity and to be able to attend the races as an owner really is very exciting. I can’t wait.”
He is part of the Petrushka Syndicate which owns two yearlings, a filly by Fastnet Rock (pictured), out of Labisa who will be trained by Richard Hannon and a colt by Danehill Dancer, out of Riberac, who will be trained by Sir Michael Stoute.
Highclere Thoroughbred Racing was started in 1992 by the son of the late seventh Earl of Carnarvon, Harry Herbert, whose family home is Highclere Castle.
Mr Herbert said, “It is great to have Hugh on board as one of our owners at Highclere and we wish him all possible luck with these two lovely yearlings.”