Jason Charles Leffler (September 16, 1975 – June 12, 2013) was an American professional open-wheel and stock car racing driver.

Leffler began racing in the open-wheel ranks, competing in the 2000 Indianapolis 500 before moving to primarily NASCAR competition.

Leffler's success also caught the attention of Joe Gibbs Racing, a team which had previously signed Tony Stewart from the USAC ranks.

After that season he moved up to the Winston Cup Series to become the driver of the #01 Cingular Wireless Dodge for Chip Ganassi Racing as the permanent replacement for Kenny Irwin Jr., who was killed in a practice crash while driving for the same team at New Hampshire in 2000.

It was a controversial decision, as Leffler performed poorly the season prior in excellent Joe Gibbs equipment in the Busch series.

After his 37th-place finish in the 2001 championship, Ganassi replaced him with Jimmy Spencer for the 2002 season and remade Leffler's former car into the #41 Target Dodge.

Leffler joined Ultra Motorsports in 2002 to drive the #2 Carquest/Team ASE Dodge Ram in place of the departed Scott Riggs and had great success early on with the team.

Haas CNC Racing, which was fielding its first full-time Cup Series team, had fired its driver Jack Sprague after the Tropicana 400.

He also was involved in a controversial finish at the Winn-Dixie 250; he was penalized by NASCAR for "over-aggressive driving" on the final lap, that saw Michael Waltrip get spun, and then Dale Earnhardt Jr. get wrecked, both by Leffler.

Shortly after his dismissal, Leffler signed a deal to re-join Joe Gibbs Racing for 2005, taking over a newly created Cup team sponsored by FedEx.

11 Chevrolet was regularly outside of the top 35 in points, meaning that it was not guaranteed a starting spot for all races; Leffler was unable to qualify for the Coca-Cola 600 because of it.

He was replaced by Terry Labonte for the two road-course races and, eventually, was fired from JGR after nineteen starts in which he failed to record a top ten finish.

He was replaced by a mix of Labonte and JGR developmental drivers J. J. Yeley and Denny Hamlin, the latter of whom took over the car full-time the following season.

Leffler would make NASCAR history on July 28, 2007 as he passed Greg Biffle with two laps remaining to win the Busch Series Kroger 200 at O'Reilly Raceway Park.

19 Toyota Camry at Pocono Raceway in early June, three days before his death; he started and parked, finishing 43rd in the event.

[8] On June 12, 2013, at 8:30 PM, Leffler was involved in a crash during a 410 sprint car heat race at the 5⁄8-mile (1.0 km) Bridgeport Speedway in Logan Township, New Jersey.

[9] Running second with a few laps left, his car suffered a front suspension failure, causing it to crash into a wall and flip several times.

When it was found that Leffler was not breathing, the rest of the race was cancelled and victory lane ceremonies did not take place.

He was transported by ambulance to Crozer-Chester Medical Center in Chester, Pennsylvania,[11] where he was pronounced dead at 9:00 PM EDT, 30 minutes after the accident.

NASCAR drivers competing in the 2013 Quicken Loans 400 had special stickers placed on their cars in honor of Leffler.

[15][16] He shared his life with live-in girlfriend Julianna Patterson; they resided in North Carolina and were engaged at the time of his death.