Jo Kimber said the thesis, which she found in Swansea's library while conducting research, contained a large section that had been copied word for word from an original and copyrighted article, with no proper acknowledgement of the source.
She also claimed that since raising the issue, her own grades for essays on her MSc course had fallen dramatically. Swansea denies there is any connection between her complaint about the thesis and her grades.
Ms Kimber said Swansea had taken so long to deal with an appeal against her grades that she would have to defer her second year and could be forced to drop out.
After an investigation, the University of Wales concluded that while there was a failure to adhere to high standards of referencing in the thesis, there was no intention to deceive. For this reason, the author was not guilty of plagiarism.
Ms Kimber condemned the conclusions as "preposterous", and said she felt they devalued her own degree.
In a letter to Swansea, she says: "It has now been established beyond doubt that the practice of copying is tolerated in Swansea. I have tried to get both Swansea itself and the University of Wales to do something about this, but neither has been willing. Having exhausted internal procedures, I have been forced to conclude that the public must be told about this decline in standards."
A University of Wales spokesman said that an investigation into the allegations had concluded there was insufficient evidence to proceed to a panel of inquiry. "We felt that it was not a case of plagiarism at all but an example of poor academic practice and overdependence on a source document," he said.
Swansea's department of social studies had been told to take action "to avoid a recurrence", he added.
The Campaign for Academic Freedom and Standards, which has been supporting Ms Kimber in her complaint, says in a letter to the University of Wales:
"The implication is clear: poor academic practice is less reprehensible than the intention to deceive."