Leffler was killed while competing in a qualifying race for the "Night of Wings" event at the Bridgeport Speedway in Swedesboro, New Jersey.
During the 25-lap race, sprint cars run over dirt or paved oval tracks. Leffler was on the fourth turn when "his 410 sprint car flipped several times down the front straightaway."
After being extracted from the vehicle, Leffler was reportedly unconscious with major injuries. He was taken to Cooper University Hospital in Camden.
He was pronounced dead in the evening.
Leffler, who began racing when was 12 years old, according to his website, returned to NASCAR dirt-racing earlier this year. He had two career wins, eight poles and 107 top 10 finishes.
NASCAR offered condolences to Leffler's family in a statement.
“NASCAR extends its thoughts, prayers and deepest sympathies to the family of Jason Leffler who passed away earlier this evening. For more than a decade, Jason was a fierce competitor in our sport and he will be missed.”
In addition, NASCAR put together a tribute video to Leffler on their website.