The 1925 24 Hours of Le Mans was the 3rd Grand Prix of Endurance, and took place on 20 and 21 June 1925. It was the last of the three races spanning 1923 to 1925 to determine the winner of the Rudge-Whitworth Triennial Cup, as well the second race of the inaugural Biennial Cup.

The Automobile Club de l'Ouest (ACO) was pleased with how the 1924 regulations had worked. They adjusted the hood-test so that all cars could do it at the same time. The start was the logical point and to stop drivers from jumping the gun they would be lined up on the opposite side of the track. When the flag fell, they would run across, put the hood up and then start the car and get away as quick as possible. This became the origin of the famous “Le Mans start” that was an institution of the race until 1969, when safety concerns led to its end.

The ACO offered a FF500 prize for the quickest to put up their hood, and the French agent for Truffault-Hartford suspension parts offered a FF1000 prize to the car leading after the first lap. The hoods now had to stay up for at least the first twenty laps (about two hours), and they had to be examined by officials for robustness before being pulled down. In addition to the 20-lap rule between fluid replenishment, no liquids could now be added after 3pm, in the final hour of the race.

Another important change was that every car had to cross the finish line and take the chequered flag to be classified. Using the cars’ average race speeds, the officials would calculate each car's estimated position on the track at the 24-hour mark. The sliding scale of target distances were again adjusted, though not as severely as 1924. The distances included the following:

This year there were a number of cash prizes awarded by the ACO and assorted sponsors for events ranging from leading at certain times to quietest and most comfortable cars. Finally, this race was the culmination of the inaugural Triennial and Biennial Cups. Only seven manufacturers' entries remained who had completed both the 1923 and 1924 races to be in contention for the Triennial Cup. There were eight entrants for the Biennial Cup. Both would be decided solely by the results in this race, not from the combined aggregate.

With the further success of the race, the ACO looked at getting more permanent facilities. They tried to purchase the properties around the pit area at Les Raineries however the prices demanded by the landowners were too high. Frustrated, the ACO instead resolved to relocate with more amenable neighbours by the hippodrome along Les Hunaudières, the main route from Le Mans to Tours. A new temporary pits, lighting and grandstands were set up on the Mulsanne Straight. The national roads board assisted by widening and sealing the track around the new pits area to assist with the new start procedure.

The international attention that had come with Bentley's win the previous year drew a much bigger list of entries. Of the 68 submitted, 55 arrived for scrutineering. But this included 15 cars from outside France. Sunbeam, AC and Austin joined Bentley from Great Britain, and there were teams from Italy (Diatto and OM) and the USA (Chrysler). There were also six different tyre companies represented at the race.

After their distance victory the previous year, Bentley returned with a full works car to support John Duff’s privateer entry and carrying the “favourites” tag. Duff had the same car as last year. The works car had an uprated 3.3-litre engine and was driven by Bentley salesman Bertie Kensington-Moir and Dudley Benjafield, a Harley St doctor and Bentley racer at Brooklands.