Penglais Farm will have a total of 1,000 rooms, but only 700 will be ready this month to welcome students.
The university said developer Balfour Beatty confirmed the remaining 300 rooms will be ready during the 2015-16 academic year.
Balfour Beatty has been asked to comment.
The unfinished rooms have not been let to students.
Hundreds of rooms at a £45m student halls development at Aberystwyth University will not be ready for the new term.