The 2017 Breeders' Cup World Championships was the 34th edition of the premier event of the North American thoroughbred horse racing year. The 13 races, all of which were Grade I, took place on November 3 and 4 at Del Mar Racetrack in Del Mar, California and were telecast by NBC and NBC Sports. The Breeders' Cup is generally regarded as the end of the North America racing season, although a few Grade I events take place in later November and December.

The highlight of the event was Gun Runner's victory in the Classic, likely wrapping up Horse of the Year honors. Forever Unbridled, Roy H and World Approval likely wrapped up divisional Eclipse Awards with victories in the Distaff, Sprint and Mile respectively. The event featured many longshot winners, including Bar of Gold at 66–1, the second largest payout in the event's history.

The Breeders' Cup Challenge is a series of races that provide the winners of designated races with automatic "Win and You're In" berths in a specified division of the Breeders' Cup. Fifty pre-entrants in the 2017 Breeders' Cup qualified via the Challenge series, which was particularly important as seven of the Breeders' Cup races were oversubscribed. A maximum of 14 horses (12 in the Turf Sprint) are allowed to start in each race. Winners of the Challenge races are given automatic entries, while other pre-entries are ranked by a points system and the judgement of a panel of experts.

Five winners of Challenge series races went on to win their respective division at the Breeders' Cup:

The Breeders' Cup organization announced in 2014 that Del Mar would host the event for the first time in 2017. Del Mar is best known for its prestigious summer meet, highlighted by the Pacific Classic. However, its seating capacity is notably lower than most other tracks at which the Breeders' Cup is run such as Santa Anita and Churchill Downs. As a result, Del Mar limited the attendance to 37,500 for both Friday and Saturday (compared to the 2016 figures of 45,000 on Friday and over 70,000 on Saturday at Santa Anita). Despite this, ticket revenues were higher than in 2016 because Del Mar designed many premium seating options.

To accommodate the event, Del Mar widened its turf course to be able to handle 14 horses. Because of the dimensions of track, the distances for two races were altered: the Turf Sprint was shortened to 5 furlongs (normally 6+1⁄2) and the Filly & Mare Turf was shortened to 1+1⁄8 miles (normally 1+1⁄4). Del Mar built temporary seating areas and upgraded the television monitors and video board. In response to unusually heavy fatalities during its 2016 summer meet, Del Mar revamped the main track to increase the banking on the turns and even out the surface. There were no life-threatening injuries during the 13 Breeders' Cup races.

For 2017, the Breeders' Cup announced that they would be performing out-of-competition testing on roughly 90% of horses (up from 40%-60%), including all North American Breeders' Cup Challenge race winners and at least one horse from each trainer. They also made a rule change that required horses who have been administered anabolic steroids for therapeutic purposes to miss six months of racing. The previous rule required a 60-day delay, with some state-to-state variation in how the rule was administered. This inconsistency had resulted in the disqualification of Masochistic from a second-place finish in the 2016 Sprint.

On October 25, the Breeders' Cup announced that 187 horses had been pre-entered for the event, including nine returning or former champions: Arrogate, Highland Reel, Drefong, Oscar Performance, Lady Eli, Champagne Room, Queen's Trust, Finest City, and Mongolian Saturday. The early entries included a record 46 horses from Europe, 14 of which were trained by Aidan O'Brien.