The most recent mishap occurred during Starliner's first crewed test flight in 2024, carrying NASA astronauts Butch Wilmore and Suni Williams. Several thrusters on Starliner's propulsion system shut down during its approach to the ISS.
The US space agency and Boeing had been discussing the future of the Starliner program for months following the blunder that left the crew on the ISS for nine months.
A Boeing spokeswoman said the company remains committed to the program.
Boeing's contract under NASA's Commercial Crew Program, previously worth $US4.5 billion ($A7 billion), envisioned six post-certification operational flights carrying astronauts to and from the ISS.
The latest modification reduces the number of Starliner flights to four, including up to three astronaut flights and an uncrewed cargo flight in April 2026, NASA said in a statement on Monday. Two additional flights are optional, the agency said.
Those modifications reduced the value of Boeing's contract by $US768 million to $3.7 billion ($A5.7 billion), a NASA spokesperson said.
NASA picked Boeing and SpaceX in 2014 to each build and fly spacecraft that can carry US astronauts to and from the ISS, seeding development of two capsules that can serve as backups to one another.
While SpaceX's Dragon capsule has become NASA's primary astronaut ride since its first crewed mission in 2020, Boeing's Starliner has been dogged for years by delays, technical faults and cost overruns.
"Safety remains our highest priority as we focus on the Starliner-1 mission, which incorporates our findings and learnings from previous flight tests and testing this fall," the Boeing spokeswoman said.
