The Spanish reporters were prevented from entering despite the fact they were all wearing the traditional black abaya and veil, according to one of the journalists, Esther Bazan, of Spain's SER radio.
Saudi authorities said the university was an all-male institution and women were not allowed.
Last minute decision
The Spanish delegation and reporters travelling with them were not informed about the ban until Sunday night, but tried to enter the university anyway on Monday.
A justice ministry spokeswoman in Madrid said: "Yesterday, at the last minute, we were told women couldn't enter the university.
He cancelled because the none of the women were allowed to be there."
She said a written copy was given to students instead. Graduates of Al-Imam Muhammad Ibn Saud university work in the government, mosques, courts and in the religious police force which patrols all Saudi cities and towns to enforce Islam-oriented public order.
Last March, Britain's Prince Charles became the first non-Muslim foreign dignitary to visit the university.
The lecture was to have closed a two-day visit by Lopez Aguilar, who is stepping down from the justice post to run for office in his native Canary Islands region.