Earlier, it had said it had expelled her for disturbing classes through "a flagrant lack of respect for ethical principles, academic dignity and morality".
Before she was readmitted, Ms Arruda said she had never been warned about her dress, the private Agencia Estado news agency reported.
Surrounded by several lawyers, she told a press conference she had been humiliated by the experience.
The education ministry had called on the university to explain its decision, while the minister for women's rights criticised it for "this total intolerance and discrimination".
Ms Arruda vehemently denied a claim by a university lawyer that she had raised her dress, stopped to pose for photos, and tried to attract attention by choosing the longest route to get to a class, reports said.
