St Benet's Hall trustees have decided female students will be able to apply from this autumn.
Prof Werner Jeanrond, master of St Benet's, said it was "self evident" both women and men should be admitted.
Last week Oxford University announced its first female vice-chancellor, Louise Richardson.
All-male and all-female colleges have been switching to admitting both men and women since the early 1970s.
This process of colleges for undergraduates becoming co-educational is now complete.
St Benet's is one of Oxford's "private permanent halls", which tend to be smaller, specialist institutions.
It has links to the Catholic Benedictine religious order, but admits students of all faiths.
Prof Jeanrond said the barrier to admitting women had been practical, with the hall needing to find extra space for accommodation, but a suitable building had now been found.
"Some people would love Oxford to be a museum - but I don't," said Prof Jeanrond.
"It depends on how you view tradition. Is it something that you contribute to or something that is an exhibit in a museum?"
The last all-male academic institution at Oxford University admitting undergraduates has voted unanimously to admit female undergraduate students.