The Students' Union Superannuation Scheme (SUSS), in which the National Union of Students is the principal employer, henceforth will be closed to all future accrual.
A defined-contribution scheme - in which the value of a pension is based on the value of the stock market at the time of retirement - is likely to be provided instead.
The NUS blamed low investment returns and increased longevity for the scheme's current £50 million deficit.
But Ben Thomas, senior national education officer for Unison, said: "We recognise that the pension scheme had some financial difficulties, but the replacement schemes are mostly inadequate and are likely to represent a massive cut in future pensions for student union staff."
Mr Thomas added: "We will be consulting on what action we intend to take."
The move is the latest in a series of pension cuts in higher education, potentially affecting all categories of staff.
Matt Hyde, chief executive of the NUS, said that the potential liabilities for students' unions if other employers withdrew from the multi-employer scheme posed "a significant risk to the sustainability of individual students' unions".
He said that the decision to close the final-salary scheme "was not taken lightly, but it was felt to be the only option in order to protect those with accrued benefits and to safeguard the future of member students' unions".
Mr Hyde added: "With the SUSS trustees, we are now working to provide fair and sustainable pension arrangements and death-in-service benefits for the employees of our member students' unions."