The move was not unexpected because Sprint has been struggling in the wireless phone market against such rivals as Verizon and AT&T.
Sprint in recent months has eliminated more than 2,000 jobs and undergone senior-management changes.
The company cited "a need to focus more directly on its core business priorities" for its decision to end the sponsorship of what is now stock-car racing's Sprint Cup Series.
"We understand significant changes within Sprint and the highly competitive business environment" led to Sprint's move, NASCAR spokesman Brett Jewkes said in a statement.
NASCAR is "very confident of moving forward in 2017 with an outstanding new partner," he said.
The sponsorship began in 2004 when Nextel signed a 10-year deal for what was then the Nextel Cup Series, a deal reportedly valued at $700 million. The name changed to the Sprint Cup Series in 2008 after Nextel and Sprint merged.
Sprint later signed a three-year extension that takes the contract through 2016.
NASCAR's second-level series, currently called the Nationwide Series under sponsorship with Nationwide Insurance, also is getting a new title sponsor starting next year. Internet provider Xfinity, a unit of Comcast Corp., signed a 10-year deal reportedly valued at $200 million in September to replace Nationwide next season. The series will be called the NASCAR Xfinity Series.