The crash happened shortly before 1:30 a.m. on the ramp from Interstate 95 north to Route 128 north in Canton.
State Police said a 28-year-old Randolph man was driving a 2012 Toyota Camry at the time.
“The driver, for reasons still under investigation, failed to negotiate the ramp’s right bearing curve and drove off the left edge of the road onto the grass shoulder,” State Police said.
The car then hit a guardrail before going across the road, striking another guardrail.
The man’s arm was amputated in the crash. He was taken by ambulance to Boston Medical Center with life-threatening injuries.
A 23-year-old Randolph woman was a passenger in the car. She was not injured in the crash.
The investigation is currently ongoing.