The grant is being cut from Â£6.7m to Â£5m by 2020, but most of S4C's funding now comes from the BBC licence fee.
The debate, in the Commons, will be led Carmarthen West and South Pembrokeshire Conservative MP Simon Hart.
In December, Broadcasting Minister Ed Vaizey said S4C was well funded, with a "guaranteed income" of Â£90m a year.
MPs will debate later how Welsh language TV channel S4C should be funded in the future, as its grant from UK ministers continues to shrink.