Who Owns the Publishing Rights of my Ph.D. Thesis?
Did you sign an agreement to transfer copyright to someone else?

Does your university policy (example) or employment contract specify that someone else holds the copyright to your thesis?

If the answer to both questions is "no" then you, as the author of the thesis, hold the copyright.

In the US, most university students retain the copyright for their thesis. Often they are required to grant the university and/or ProQuest a non-exclusive license to distribute the thesis, but without giving up copyright.